Finally... the upgrade is complete. Even though I did what I thought was rather extensive testing for the past three days, there still were several rather major bugs... I suppose the amount of modification to Mediawiki was more extensive than I thought. The good news is that I've fixed all the bugs I found after the upgrade.
*Please* report all bugs you find to the forum, regardless of how small. If you can, please test IMSLP while logged out too (many bugs hide there because generally anonymous users don't register on the forums either...).
I've done way too much today (coded for more than 12 hours, and tired as hell haha); I'll finish up everything else tomorrow (including documentation of new features, etc).