Jamieson, Fausset and Brown on Genesis 17:5

5. but thy name shall be Abraham--In Eastern countries a change of name is an advertisement of some new circumstance in the history, rank, or religion of the individual who bears it. The change is made variously, by the old name being entirely dropped for the new, or by conjoining the new with the old; or sometimes only a few letters are inserted, so that the altered form may express the difference in the owner's state or prospects. It is surprising how soon a new name is known and its import spread through the country. In dealing with Abraham and Sarai, God was pleased to adapt His procedure to the ideas and customs of the country and age. Instead of Abram, "a high father," he was to be called Abraham, "father of a multitude of nations" (see Re 2:17).