So I now have WoW: Burning Crusades. Installation went smoothly, then I tried to run it.
First off, since the patches I had from before only worked their magic on the regular game's data, it had to install them again. No big deal -- they're still on my hard drive.
So I get up to version 2.4.2 when, uh oh, somehow some data was corrupted. I ran the repair tool and found out that the corruption was "serious." When WoW suffers from "serious" corruption, it rolls the game back to BC.0. Long story short, I'm currently waiting for 2.3.0 to install. Argh!
I have one of those sinking feelings, too, like it's going to do that again. Now I'm glad I kept the WoW installer around...