iTunes Match questions regarding songs marked as "Uploaded"
I have over 5000 tracks which have been uploaded, whenever I add new music, Match is updated, and it determines whether to upload it or match it.

I was interested in this also, so I created a smart playlist for the kind set to MPEG audio and within the playlist I display the iTunes Match columns.

Recently I checked this list, and instead of all of them displaying Uploaded, a couple of hundred had changed to Matched, so I deleted them locally and re-downloaded the matched version.

So in answer to your question, yes, they are sometimes updated, and there's nothing you need to do other than occasionally update match by adding new music, or manually from the menu, and then check your smart playlist.