Macworld's Jason Snell describes in this article how to upgrade tracks with iTunes Match to 256kbps AACs. iTunes Match will allow you to re-download these types of files even if the matched tracks are lesser quality. To gather these lesser-quality matched-tracks together, Jason describes how to create a Smart Playlist with the correct criteria. I've created that Smart Playlist and exported it and have included it in a script which, when run, will add that Smart Playlist to your iTunes for you. Here is the direct download link. Just download and double-click the script; make sure iTunes is running. Be sure to read Jason's article to understand the rest of the details.