Kommentar zu Bereschit 17:5

כי אב המון גוים FOR A FATHER OF A MULTIDUDE OF NATIONS — An acrostic on his name [אב (ר) הם]. But the ר which was previously in his name — denoting that he was the father (chieftain) only of אב (א) רם] ארם] which was his native country (cf. Genesis 21:7 with verse Genesis 21:10 of the same chapter), whilst now he became father of the entire world — nevertheless was not taken out of his name. For we find also, that the 'י in Sarai’s name continued to complain to God that it had been removed from her name when this was changed to Sarah, until it was placed as an additional letter into the name יהושע, as it is said, (Numbers 13:16) “And Moses called Hoshea (הושע) the son of Nun, Joshua (יהושע)” (Genesis Rabbah 47:1).

והיה שמך אברהם, "henceforth you name will be Avraham. Immediately."

ולא יקרא שמך עוד אברם, your name will no longer be Abram, but Abraham. Bereshit Rabbah 44,10 already stated that when Abram cited that he had seen in his horoscope that he would not father any children, G'd agreed with him and told him that he was correct, that as Abram he would not father children but that as Abraham he would. The same would apply to Sarai as compared to Sarah. This Midrash is very difficult since the Torah reports Abram as siring Ishmael, not Abraham!