Probably better to compare the promises as they are given in both passages:
Genesis 12
King James Version
12 Now the Lord had said unto Abram, Get thee out of thy country, and from thy kindred, and from thy father's house, unto a land that I will shew thee:
2 And I will make of thee a great nation, and I will bless thee, and make thy name great; and thou shalt be a blessing:
3 And I will bless them that bless thee, and curse him that curseth thee: and in thee shall all families of the earth be blessed.
The Lord is presenting the Gospel, and the blessing for all families of the Earth is a reference to Christ.
Genesis 15
King James Version
1 After these things the word of the Lord came unto Abram in a vision, saying, Fear not, Abram: I am thy shield, and thy exceeding great reward.
2 And Abram said, Lord God, what wilt thou give me, seeing I go childless, and the steward of my house is this Eliezer of Damascus?
3 And Abram said, Behold, to me thou hast given no seed: and, lo, one born in my house is mine heir.
4 And, behold, the word of the Lord came unto him, saying, This shall not be thine heir; but he that shall come forth out of thine own bowels shall be thine heir.
5 And he brought him forth abroad, and said, Look now toward heaven, and tell the stars, if thou be able to number them: and he said unto him, So shall thy seed be.
6 And he believed in the Lord; and he counted it to him for righteousness.
Here is another aspect of the promise that speaks of literal descendants and their number.
If Abraham had refused to sacrifice Isaac it would have shown he did not believe God's promise, hence it would not have been given as an example of faith/belief. So it isn't a relevant "hypothetical" question.
So I would say that Abraham's faith didn't change the promise God made. Abraham was just part of that promise, and understood it in physical terms (that he would receive a son from a wife beyond bearing).
The promise is irrevocable because of God, not Abraham:
Hebrews 6:17-19
King James Version
17 Wherein God, willing more abundantly to shew unto the heirs of promise the immutability of his counsel, confirmed it by an oath:
18 That by two immutable things, in which it was impossible for God to lie, we might have a strong consolation, who have fled for refuge to lay hold upon the hope set before us:
19 Which hope we have as an anchor of the soul, both sure and stedfast, and which entereth into that within the veil;
God bless.