Well, once the words of preference were such things as "idiot", imbecile" and "moron", and there were great debates on the appropriate dividing line between them. Those were abandoned for the then politically-correct term, "retarded", meaning delayed. That was abandoned when it (or its derivatives) became terms of abuse (normally not directed toward those who didn't fit the medical definition). The newer expression "developmentally disabled" is being replaced with "developmentally delayed"...and that is being overtaken by "intellectually disabled".

Jumping into this with both size 13 boots (and I think this is principally a US issue), we once used the word Negro (with or without capitol letter) to describe any person who had one ancestor of African origin, regardless of how far removed. The word, with a slight noun shift, is the Spanish word for black. Polite people said "Colored" instead - and we know what impolite people used. Both of these were replaced at the beginning of the PC era with Black, which was then replaced by African-American. Strangely enough, during the last presidential election, there was some discussion on whether or not Mr. Obama, with one American parent of Northern European descent and one African parent, was entitled to be called an African-American!