Copy & Paste between tags

My problem: I have an iPod, and a lot of "various artists" albums, all tagged with the artist on each track. Apple for some stupid reason, has decided to sort coverflow album art by artist, and repeats the cover for each album many times (as many as there are artists). Compilation albums basically break coverflow.

To solve this, I want to tag all the files "various artist" in albums like this, but I don't want to lose the info that is currently in that tag. So I'd like to copy it to the comments tag or some other appropriate tag. How do I select many mp3 files and copy the info from one tag to another?

Nevermind. I was actually able to solve my original problem without editing the mp3 tags. I simply needed to mark each track a "compilation" in iTunes. That fixes my sorting dilemma.