iTunes Plus Upgrades Go Singular
If you go to the iTunes Plus upgrade page via the Upgrade to iTunes Plus link on the iTunes Store's Home Page, there's a Buy All button for a one-step upgrade, or you can click Buy buttons next to individual albums or tracks in your library.
Admittedly, if you have a ton of songs, doing them one at a time is going to take a while. You sure you don't want to do it all at once?
Anyway, prices are still the same, so it can still cost you an arm and a leg if you need to upgrade a lot of tracks. It's $0.30 per track, about 1/3 the cost of an album to upgrade an album, and $0.60 for a music video.
For those of you early adopters who upgraded your whole catalog and only did so because you had no choice, don't expect to be able to talk them into a refund. According to the iTunes Store Terms of Sale, all purchases made on the iTunes Store are final (a friend already tried, and Customer Support was not too helpful).