Abram and Sarai
Abraham and his wife / half sister Sarah (they had the same father, but different mothers i.e. Genesis 20:12) were known by those names for only the latter part of their lives - after their righteous character was made evident over many years. The Lord changed Abram's name to Abraham when he was 99 years old (Genesis 17:1,5); He changed Sarai's name to Sarah when she was about 90 years old (Genesis 17:15,17). Nevertheless, those are the names that they are most well-known for in Bible History because it was The Lord's purpose in renaming them that made them, and their descendants, famous.
"a father of many nations have I made thee ... she shall be a mother of nations"
Abram (pronounced in Hebrew awb-rawm) means high father, while Abraham (pronounced in Hebrew awb-raw-hawm) means father of a multitude i.e. a righteous elder was chosen to become the progenitor of many nations.
"And when Abram was ninety years old and nine, The Lord [see YHVH, Adonai, Jehovah, LORD] appeared to Abram, and said unto him, I am the Almighty God; walk before Me, and be thou perfect. And I will make My covenant [see also Covenant and Testament] between Me and thee, and will multiply thee exceedingly."
"And Abram fell on his face: and God talked with him, saying, As for Me, behold, My covenant is with thee, and thou shalt be a father of many nations. Neither shall thy name any more be called Abram, but thy name shall be Abraham; for a father of many nations have I made thee."
"And I will make thee exceeding fruitful, and I will make nations of thee, and kings shall come out of thee. And I will establish My covenant between Me and thee and thy seed after thee in their generations for an everlasting covenant, to be a God unto thee, and to thy seed after thee. And I will give unto thee, and to thy seed after thee, the land wherein thou art a stranger, all the land of Canaan, for an everlasting possession; and I will be their God." (Genesis 17:1-8 KJV)
Sarai (pronounced in Hebrew saw-rah-ee) means noble woman, while Sarah (pronounced in Hebrew saw-raw) means princess. As with Abram's name change to Abraham, Sarai's singularly noble name was multiplied in meaning to Sarah.
"And God said unto Abraham, As for Sarai thy wife, thou shalt not call her name Sarai, but Sarah shall her name be. And I will bless her, and give thee a son also of her [see also Isaac's Faith And Obedience]: yea, I will bless her, and she shall be a mother of nations; kings of people shall be of her." (Genesis 17:15-16 KJV)
Fact Finder: Was Jesus Christ a direct descendant of Abraham and Sarah through their child Isaac?
Matthew 1:1-2, Luke 3:34