The original replacement theologians were the Jews who immediately forsook God's inclusivity in the promises of the Abrahamic covenant.
[Gen 15:5-6 NASB] 5 And He took him outside and said, "Now look toward the heavens, and count the stars, if you are able to count them." And He said to him, "So shall your descendants (seed, singular - meaning Jesus) be." 6 Then he believed in the LORD; and He reckoned it to him as righteousness.
[Gen 17:4-8 NASB] 4 "As for Me, behold, My covenant is with you, And you will be the father of a multitude of nations. 5 "No longer shall your name be called Abram, But your name shall be Abraham; For I have made you the father of a multitude of nations. 6 "I will make you exceedingly fruitful, and I will make nations of you, and kings will come forth from you. 7 "I will establish My covenant between Me and you and your descendants (seed, singlular) after you throughout their generations for an everlasting covenant, to be God to you and to your descendants (seed, singular) after you. 8 "I will give to you and to your descendants (seed, singular) after you, the land of your sojournings, all the land of Canaan, for an everlasting possession; and I will be their God."
Paul explains this covenant of faith in depth in Romans 4:
[Rom 4:9-13, 16-17 NASB] 9 Is this blessing then on the circumcised, or on the uncircumcised also? For we say, "FAITH WAS CREDITED TO ABRAHAM AS RIGHTEOUSNESS." 10 How then was it credited? While he was circumcised, or uncircumcised? Not while circumcised, but while uncircumcised; 11 and he received the sign of circumcision, a seal of the righteousness of the faith which he had while uncircumcised, so that he might be the father of all who believe without being circumcised, that righteousness might be credited to them, 12 and the father of circumcision to those who not only are of the circumcision, but who also follow in the steps of the faith of our father Abraham which he had while uncircumcised. 13 For the promise to Abraham or to his descendants that he would be heir of the world was not through the Law, but through the righteousness of faith. ... 16 For this reason [it is] by faith, in order that [it may be] in accordance with grace, so that the promise will be guaranteed to all the descendants, not only to those who are of the Law, but also to those who are of the faith of Abraham, who is the father of us all, 17 (as it is written, "A FATHER OF MANY NATIONS HAVE I MADE YOU") in the presence of Him whom he believed, [even] God, who gives life to the dead and calls into being that which does not exist.
[Gal 3:16 NASB] 16 Now the promises were spoken to Abraham and to his seed. He does not say, "And to seeds," as [referring] to many, but [rather] to one, "And to your seed," that is, Christ.
[Gal 3:6-9 NASB] 6 Even so Abraham BELIEVED GOD, AND IT WAS RECKONED TO HIM AS RIGHTEOUSNESS. 7 Therefore, be sure that it is those who are of faith who are sons of Abraham. 8 The Scripture, foreseeing that God would justify the Gentiles by faith, preached the gospel beforehand to Abraham, [saying,] "ALL THE NATIONS WILL BE BLESSED IN YOU." 9 So then those who are of faith are blessed with Abraham, the believer.
[Isa 54:1 NASB] 1 "Shout for joy, O barren one, you who have borne no [child;] Break forth into joyful shouting and cry aloud, you who have not travailed; For the sons of the desolate one [will be] more numerous Than the sons of the married woman," says the LORD.
[Gal 4:22-28 NASB] 22 For it is written that Abraham had two sons, one by the bondwoman and one by the free woman. 23 But the son by the bondwoman was born according to the flesh, and the son by the free woman through the promise. 24 This is allegorically speaking, for these [women] are two covenants: one [proceeding] from Mount Sinai bearing children who are to be slaves; she is Hagar. 25 Now this Hagar is Mount Sinai in Arabia and corresponds to the present Jerusalem, for she is in slavery with her children. 26 But the Jerusalem above is free; she is our mother. 27 For it is written, "REJOICE, BARREN WOMAN WHO DOES NOT BEAR; BREAK FORTH AND SHOUT, YOU WHO ARE NOT IN LABOR; FOR MORE NUMEROUS ARE THE CHILDREN OF THE DESOLATE THAN OF THE ONE WHO HAS A HUSBAND." 28 And you brethren, like Isaac, are children of promise.
You see those who originally followed Abraham were the original replacement theologists. They replaced the truth that Abraham was the father of many nations through faith in the gospel of Jesus Christ with the lie that he was the father of one nation - the Jews. But Abraham was an uncircumcised gentile when he was made righteous for his faith in the gospel of Jesus Christ. God promised that uncircumised, righteous gentile that all of his seed (from many nations) would inherit that righteousness he was given (Christ's righteousness).
Often when I read scripture and see both Israel and Judah mentioned in the same sentence, I think the proper understanding is that those descendants of the faith of Abraham are Israel and those who are physical descendants are Judah or sometimes "Jacob". For example:
[Gen 49:24 NASB] 24 But his bow remained firm, And his arms were agile, From the hands of the Mighty One of Jacob (From there is the Shepherd, the Stone of Israel),
[1Ch 16:13, 17 NASB] 13 O seed of Israel His servant, Sons of Jacob, His chosen ones! ... 17 He also confirmed it to Jacob for a statute, To Israel as an everlasting covenant,