For me, one word (you might have guessed) - Re-Rip. But to clarify, I have a good sized (7000 tracks or so) library imported from MusicMatch, and I'm slowly converting them with MC using LAME with new and better bitrates and mp3 quality options. The current suggested techniques are workable but time-consuming. For me it's usually easier to simply rip the disc again and delete the old tracks. I'm personally less concerned about the usage stats, so that much is generally OK to lose. However, I'm particular about the way they're tagged, so I tend to edit the tags before re-ripping even if they're found in YADB, and I have a fair number of obscure old CDs that aren't found in the media database.
What I'd like to see is an alert that pops up when I press the Rip button (or at Auto-Rip time. etc.) asking me if I want to Re-Rip one of the albums in my current library. If there are multiple matches, you could present them in a manner precisely like you present options from YADB. In fact, you could present matches from the local library in the same dialog with any online matches. How difficult would it be to scan the current Library for matching albums before hitting the online YADB? That's not a rhetorical question - I realize there might be some difficulties in assembling album candidates, especially if the local Library database is based on tracks. If Media Center is multi-threaded, you could start that process as soon as you detect that a disc is inserted.
Anyways, that's my request! Otherwise I'm really happy with MC. Thanks..