I'd disclose it---certainly if it changed the apparent serial number. Someone might want to do something that depends on the date of the camera, for instance; maybe they need to replace some weird little part that changed from one year of production to the next, or whatever.

In general I just think full disclosure is the safest way to do any transaction. It eliminates a lot of opportunities for later ill-feeling if someone's assumptions turn out to be wrong. If a buyer decides to open your Olympus lens for whatever reason, maybe just for their own interest, and discovers that it looks like hell inside, you don't want the result to be an annoyed post that says "This guy sold me a mangled lens", right?