I noticed it mostly when they
1) Left for college
2) Graduated college
3) Earned their first advanced degree; or most commonly
4) When their new wife doesn't want them called by the name that the entire world knew them by for the 25 years before meeting her.
Sorry Billy. You're always going to be Billy to me no matter how badly your wife wants to try and shed that good-time-beer-drinking persona that went with your name for years. I have no fucking clue who William is.