I am going to take a very cautious approach with deleting and re-downloading to 'upgrade' my library. So far I have done a few tests and have already found some problems, such as wrong artwork and 'remasters' that sound worse than the version I had; for example Electric Ladyland by Jimi Hendrix. I would recommend taking it slow and doing an album at a time, at least at first. Or if you are going to go for it and mass-upgrade everything eligible that is under 256k bitrate, make sure to tell iTunes not to delete the old files so you can restore them if you need to. If you delete the matched iTunes versions and the old ones are still there I believe they re-associate, it worked for me with the Hendrix album.