I have been downloading iTunes music for 10 or 11 years and a lot of my purchases were early purchases when it was 128K DRM files. Also I had a bunch of CD's I ripped at the same rate, not to mention those CD had some pretty good scratches, and when ripped, produced some shitty M4As. So if you pay the $25 for a year of iTunes Match, you can do a smart playlist and find all these files and delete them then redownload them from Apple in a perfect 256Kb DRM free version. I was able to replace 1500+ files for $25, if I paid the $.30 per file Apple was offering before iTunes Match it would have cost me $450+. So Just wanted to pass this along.
http://www.macrumors.com/2011/11/15/quickly-upgrade-all-itunes-matched-songs-to-256kbps/