TL;DR: The alteration was probably which of her suitors Renisenb chose to marry.
What Christie says in her autobiography is that the alteration affected “one point of my denouement” and that she “would like to rewrite the end of it”:
Stephen [Glanville] argued with me a great deal on one point of my denouement, and I am sorry to say that I gave in to him in the end. I was always annoyed with myself for having done so. He had a kind of hypnotic influence about that sort of thing: He was so positive himself that he was right that you couldn't help having doubts yourself. Up to then, on the whole, though I have given in to people on every subject under the sun, I have never given in to anyone over what I write.
If I think I have got a certain thing right in a book—the way it should be—I'm not easily moved from it. In this case, against my better judgment, I did give in. It was a moot point, but I still think now, when I reread the book, that I would like to rewrite the end of it—which shows that you should stick to your guns in the first place, or you will be dissatisfied with yourself. But I was a little hampered by the gratitude I felt to Stephen for all the trouble he had taken, and the fact that it had been his idea to start with.
Agatha Christie (1977). An Autobiography, p. 484. New York: Dodd, Mead.
Based on this, the alteration can’t have been the identity of the murderer. This wouldn’t be described as “one point of the denouement”, it is the major plot point of the novel, and to change the murderer would require more than rewriting “the end of it”—it would require adjustments throughout the book, switching clues, motives, red herrings, and misdirections.
However, in the last chapter of Death Comes as the End, the protagonist Renisenb has to make a choice between two suitors, the “handsome young” scribe Kameni, who came from the north with Nosfret and who wants to take her away with him, and the older Hori, “her father’s man of business and affairs”, whom she has known from her childhood:
Hori smiled at her.
“Yes, sometimes. Kameni and I, Renisenb. Both of us, I think, are as you believe we are. Kameni and I...”
He said the last words with significance, and suddenly Renisenb realized that she stood at a moment of choice in her life.
Hori went on:
“We both love you, Renisenb. You must know that.”
[…] And the choice suddenly presented itself to her in the simplest terms—the easy life or the difficult one. She was strongly tempted then to turn and go down the winding path, down to the normal, happy life she already knew—that she had experienced before with Khay. There was safety there—the sharing of daily pleasures and griefs, with nothing to fear but old age and death…
Agatha Christie (1944). Death Comes as the End, chapter 23. New York: Dodd, Mead.
This choice, of which suitor to marry, is compatible with the description “one point of the denouement”, and for Renisenb to take the other choice would only require rewriting the ending of the book.