Almost "everything period" with a few exceptions. Anything you bought before the switch that has been re-released as "iTunes Plus" was upgradeable to the higher bitrate for some number of pennies per song. Only really obscure stuff wasn't converted. In my collection I have maybe 5-6 songs that never got "upgrades" available and that was out of a couple hundred. I think it cost me like $35 to get the iTunes Plus versions.
"And and if you start to bleed, stop wiping." -whm1974