iTunes Match "upgraded" one of my songs with a glitchy version. Now what?
My understanding (although not precise science) so far with Match knows the song signature and uses this in conjunction with the track length to match against their database. 

This would explain why some songs get replaced with explicit versions on Match while renaming one song to another (incorrect) does not trick Match into thinking it is a new song. You can test both quite easily 


the former by deleting a non explicit matched song (remembering track length) and pulling it back from iCloud
the latter by ripping a CD without any info into Track01...02 etc and have it matched correctly.


all the above is somehow tied with Genius. 

With the above assumption, to keep your proper version try padding out the length, add noise, or combine the track with the next (if it belongs to an album) and it should cause enough difference to be uploaded instead of being matched.