Yes, but "implicit" is just that: implicit. It is not a clear affirmation of teh Trinity, suggestive maybe, but not definite.
It is atrongly suggestive to the point where modern dynamic equivalence based translations like the NIV would take a similiar liberty with the text.
The Comma Johanneum simply makes the implicit reference explicit.
Later additions or subtractions from Scripture are always suspect in my book. They mean someone has tampered with or bludgeoned the original text to suit their biases.
So you would leave out the vital Gospel message of the Adultery pericope, and have our Lord not say "Let he who is without sin cast the first stone?"
If the addition is divinely inspired, then what is the situation with the original, which omitted that passage? Was it also divinely inspired and then, for some reason, God decided it wasn't good enough and felt something else needed be added?
Here is what I would say to that: the Comma Johanneum can be viewed as divinely inspired because the Trinity is a true doctrine of the Apostles, and an unknown pious scribe added it as a gloss to the Latin manuscripts of the Vetus Latina in order to make the Trinitarian reference more explicit, probably around the time of Paul of Samosata, the decadent bishop of Antioch, who was the first to really attack the Trinity, a few decades before Arius.
You have to understand how divine inspiration works. The Church, as a whole, is protected by God fro, the gates of Hell (Matthew 16:18), and the Church wrote the New Testament, and later edited it. At firat, the Church did not even have a New Testament; the Gospel was delivered orally, which is why the Pauline epistles, the oldest part of the New Testament, refer to it in the singular. Then, so that this oral history would not be lost, Apostles Matthew and John, and the scribes Luke and Mark, who had previously travelled with Ss. Paul and Peter (and the Cenacle was in St. Mark's house) wrote down from memory their experiences and what they had heard from their fellow Apostles, lest the oral history not be altered. Some other people also wrote pious expressioms of their understanding of the faith, like the Shepherd of Hermas.
I believe there were many more of these accounts than we presently have, but most are lost; several of them I believe were acquired by Gnostics and mutilated to suit the Gnostic heresy, for example, the Gospel of Thomas, which reads like a Synoptoc but has clear Gnostic interpolations, or the Odes of Solomon (one Patristic figure says the Gospel lf Thomas was written by Thomas the wicked disciple of Mani; I used to think he was talking about the sayings gospel recovered at Nag Hammadi. but I realized he was referring to the very perverse Infancy Gospel of Thomas.
There is an implicit error of Process Theology in your statement. God does not change His mind; when the Old Testament uses language like this, it is metaphorical and in the context of allegory. God is immutable and in all respects perfect.
The Holy Scriptures are divinely inspired because the holy prophets and apostles who composed them had direct access either through private revelation or direct experience (especially in the case of the Apostles) of God. So, the early Church started with just the Old Testament. Then the Apostles wrote the genuine inspired canon. Unfortunately, much of this was lost or corrupted by Gnostics during the Roman persecutions, although as early as St. Irenaeus, even before St. Irenaeus, we find some mention of our current canonical scripture.
When the Church finally came to be at peace during the reign of Constantinople, the process began of figuring out which books were genuine articles, written by the Apostles, and which ones were not. The main test of canonicity among the fourth century fathers was whether or not the Apostles actually wrote the book, not whether or not it was divinely inspired. The Church of Alexandria supplied fifty codices to Rome while St. Alexander of Alexandria was Patriarch, on the request of the Roman archbishop. Meanwhile, St. Constantine asked Eusebius of Caesarea, who publically admired the Emperor and died leaving unfinished a comprehensive biography of the liberator of Christians, to produce another fifty or so Bibles for use in the churches of New Rome, Constantinople, which he was building. Eusebius wrote in a letter a list of books he thought were canonical, but this list was not ultimately accepted.
A new translation of the New Testament into Syriac was made, the Peshitta, to replace the Diatessaron, the Gospel harmomy which had been dominant in the Syriac speaking churches of the Prient, but which the Fathers mistrusted owing to its authorship by Tatian, a known Gnostic heresiarch. This initially contained only those works which were most uncontroversial; all of the disputed books were omitted (they were later added to the West Syriac Peshitto, but not to the East Syriac version). St. Jerome expressed his own opinions.
Ultimately it was St. Athanasius who settled the matter, as you well know, in his 39th Paschal Encyclical. His determination as to which books in the New Testament were authentic and apostolic and which ones were not became universally accepted, and was the basis for the Decretum Gelasianum of 493, which defined permanently the canonical New Testament as far as the Church of Rome was concerned, and prohibited the use of any other books. This canon was not universally binding, but was a local canon specific to the Roman Church, but in general all churches recognize the Athanasian canon (the Ethiopians append to their Old Testament the Didascalia, one of two ancient first century manuals of church discipline, along with the Didache, the Armenians include 3 Corinthians, but do not read it liturgically).
So that is what happened. The Church wrote the New Testsment; specifically the Apostles who established the current church as the successor to the dying religion of Second Temple Judaism, which due to the destruction of the Temple became extinct during the course of its compostion. Then, after a proliferation of Gnostic psuedepigrapha and forgeries, the Church edited the Bible to determine what was divinely inspired. There are disagreements between the Apostolic churches and indeed the Protestants, particularly on the Old Testament, and these mirror similiar arguments from the fourth century. But ultimately, each local Church had to decide for itself what New Testament books were to be accepted, and the Athanasian Canon was definitive.
Now, the primary test for canonicity was apostolic authenticity; 1 Clement and the Shepherd of Hermas were excluded not because it was thought they were not divinely inspired, but because it was known their authors were not Apostles.
However, when it came to the actual Bibles that wound up being written and later printed, and used liturgically in each Church, this was a decision that had to be made. All of the Apostolic churches wound up accepting the validity of the Comma Johanneum even if they left it out of their text. Likewise, they all accepted the validity of the Adultery Pericope, and Mark 16:9-20.
It is the Church which legitimized Scripture, not vice versa; Scripture was written by and for the Church. However, because of the binding force of tradition (2 Thessalonians 2:15), once the Church came to a general agreement on what was canonical. and also, what the contents of each book should contain (specifically, all of the Apostolic churches, even the Coptic Church and the Greek Orthodox Patriarchate of Alexandria, agreed on the Byzantine Text Type or the Majority Text, and rejected the so-called Alexandrian text type or Minority Text), the matter became closed.
And while it was the early Church which gave Scripture its legitimacy, since the matter of what is Scripture is settled, Scripture now also grants the Church legitmacy, because Scripture is now a part of Holy Tradition; the Church has established a reliable, canonical Bible, with extreme difficulty, and this Bible, now that it exists, in turn authenticates the Church.
Should we be open to the possibility that Scripture maybe be modified today?
Scripture is being modified today, illicitly. Some very recent dynamic equivalence translations like the latest New International Version have, in the interests of political correctness, taken spectacular liberties with the text. The Jehovahs Witnesses published a Bible in which John 1:1 was modified to suit their dogma. And Hal Lessig and a group of postmodern and emergent theologians, many of whom had previously been involved in the Jesus Seminar under Robert Funk, produced A New New Testament, which combines a liberal, postmodern, dynamic equivalence translation of the canonical New Testament with a liberal, postmodern, dynamic equivalence translation of an assortment Gnostic scriptures, which is hillarious, because while on the one hand they distorted the canonical New Testament making it less Orthodox, on the other, they distorted the Gnostic texts making them less Gnostic and in some cases causing them to read as being more Orthodox and less bizarre (especially in the case of The Gospel of Mary Magdalene and the Apocryphon of John).
It should be stressed that several dynamic equivalence Bibles take liberties with the text which exceed by a massive degree the gloss provided by the Comma Johanneum. The Living Bible, for example. If I were to quixotically set out to compose an Orthodox Bible based on Dynamic Equivalence (which I do not believe would be very Orthodox), I could among other things translate "I and the father are one" as "I am of one substance with the Father," and my change would be far less radical than those made in many recent Bible editions.
So, in answer to your question, scripture can be modifed today. However, you asked "Maybe." So, may scripture be legitimately modified today? No, because the canon is closed and the question of what texts to use was settled in the first millenium. All of the apostolic churches, the Orthodox, the Roman Catholics and the Assyrians agreed on the Athanasian Canon, a more or less comparable Old Testament canon based on the contents of the Septuagint, and the use of the Byzantine text type. These scriptural texts were then written into the lectionaries of the churches. Because the canon is settled, one could not add a new book or delete an existing book, or add or delete verses, with amy legitimacy, because Scripture as it exists is a matter of Holy Tradition. One of the last books to be agreed as canonical was Revelations, but it contains a warning against adding or removing anything from "this book," which may be read as applying only to it, but which perhaps piety should compel us to regard all of sacred scripture with.
I myself will therefore not accept as canonical any book not included in the canon of at least one Apostolic Church, and in the case where only one included it (1 Enoch in the Ethiopian Church, for instance), I read it with extreme caution.
This does not mean that new devotional or religious literature cannot be written and regarded as divinely inspired. I believe the Epistles of Clement, Ignatius and Polycarp, and most of the Patristic corpus, is divinely inspired; I believe the Sayings of the Desert Fathers is divinely inspired, and I believe the Philokalia is divinely inspired. I also believe the ancient hymns and liturgies of the Church are divinely inspired, but subject to change. These works however are subordinate to Scripture, but they all collectively form a part of one united, divinely inspired Holy Tradition, at the center of which is the Gospel as narrated by the four evangelists, surrounded by the other books of the Holy Bible, which is truly an ikon of the Word of God (when we venerate ikons of Orthodox saints who were Church Fathers, we kiss the Bibles they are depicted as carrying).
The only possible exception I can think of, which is really hypothetical: If one became stranded followimg some global cataclysm on a remote island with a small community of people and had no Bible, I guess in that case it would be permissible to try and write down a summary of the Bible from memory.