Will iTunes Match let me "upgrade" existing tracks to higher quality or bitrate?
Yes, this is possible. Once the cloud has matched your lower bitrate song, make a final backup and delete that song from your iTunes library. You can then re-download the &quot;standard&quot; version from Apple's servers.

Jason Snell has a great writeup on an easy way to keep track of the songs by using the power of smart playlists:

Make a Smart Playlist Create a Smart Playlist with the following attributes:

Bit Rate is less than 256kbps
Media Kind is Music
Any of the following are true: (to create this conditional, option-click on the plus button in the Smart Playlist window) iCloud status is Matched, iCloud status is Purchased

Delete all those files Select all the tracks in your playlist, and then hold down the option key while pressing Delete. Be sure to not select the box that would delete them from iCloud!
Bring ’em back All those tracks should still be [in the playlist]—but now they’re marked as being available only in iTunes Match. Select all of them again, then control-click on the selection and choose Download.