Yes, the conversion of older protected tracks into purchased or matched copies is a one time deal, so you can take the iTunes Match subscription for one year, used it to release your old protected tracks, and potentially improve the quality of some of your other tracks if you have any other 128k files you want to try to upgrade to 256k. You can turn off the renewal immediately so you don't have to worry about forgetting to cancel. And if you find yourself still finding it useful there will be a reminder that the service is due to expire, such as the one I've just had.
I'd recommend you Backup your iTunes for Windows library with SyncToy and make separate archive copies of anything you replace just in case iTunes Match doesn't bring back the version you wanted it to. I have a folder called Originals & Dupes in my main iTunes folder for anything I have archived like this so it is all included in the backup. Rather than trust iTunes Match not to make any errors I have a completely separate library for working with it, which also gets around the potential issue of size, though the limits may have been raised.
tt2