Does iTunes Match keep updating? I just subscribed to iTunes Match and it's currently scanning my library.

I've got a few tracks named as 'track 1', 'track 2', and 'unknown artist'. My plan is to clean up my library (some day). But will iTunes Match be able to rescan the new names of already matched songs?

In other words, will iTunes Match re-sync those songs already synced or will it only sync newly added songs?
Yes. The iTunes Match Library icon shows up from time to time (I haven't figured out what the exact requirements are) and you can re-scan your library.

Once you changed your id3 data, select the songs, right-click and select "Add to iCloud". This initiates a re-scan in iTunes Match as well.

If you add music, it will by added to iCloud instantly.