Yes, that's essentially what happened--except for the unfortunate fact that that's not what "retronym" means. Retronyms are things like "black and white TV", and "manual transmission", where the modifier merely tells you what "TV" and "transmission" used to mean all the time. Once you start looking for retronyms you find them all over the place: "dial telephone", "snail mail", "area rug", and pretty much "analog <anything>" these days. They're a lot of fun to think about.

Unfortunately, there's no decent word that I know of for an acronym definition made up after the acronym. I believe the OED says "later rationalizations" about Perl's glosses. I think "fakeronym" is cute, but it seems like we should really make up an acronym for it. We could call it a LATE, but I don't know what that stands for yet... :-)