Just fyi: I believe [make sure this is right before you do it!] you should be able to subscribe to iTunes Match ($25/year), delete the songs from your computer,

Just fyi: I believe [make sure this is right before you do it!] you should be able to subscribe to iTunes Match ($25/year), delete the songs from your computer, then redownload them in the DRM-free format. Not free, but probably cheaper to do that than to upgrade a bunch of songs one-by-one to the DRM-free format. Then you could drop the iTunes Match subscription.