It's the same for me. In fact, even if I do what you said, sometimes the old album art comes back. The only sure way I have found for it to work is to:
1) Delete the music entirely from iTunes Match AND iTunes (keeping the files on the hard drive)
2) Turn off iTunes Match
3) Add the music back into iTunes
4) Fix the cover art (and any metadata as it has the same issue)
5) Turn on iTunes Match so the music gets uploaded again
Of course, with a large library, this takes a lot of time and effort, and takes a lot of the enjoyment out of using the product.
According to the Apple reps I have spoken to, this is a bug that Apple is aware of. They told me we are simply supposed to use workarounds like the above and wait until some time in the future when it will eventually be fixed.
I suggest anyone with this problem either "Provide iTunes feedback" here:
http://www.apple.com/feedback/itunesapp.html
Or, if you are under warranty, contact support and let them know this behavior is unacceptable in a paid product.