In my experience, if you normally auto-sync music from your iTunes library, but buy music directly from the iPhone, the iPhone purchases seem to remain on the iPhone in a separate category from synced songs. So if the purchase gets transferred to your library and then you edit the tags there, that does not seem to change the tags of the versions on the iPhone.
In the few times I've bought music directly from my iPhone, next time I re-sync to my iTunes library I go to the Music sync section of the iPhone (as seen through iTunes). Below the sync options for Playlists, Artists, Genres, and Albums, you may see "Manually added music" (or something to the effect). Here is where direct-to-iPhone purchases seem to live, and you can manually delete them. Then re-sync the album from your library to get the edited versions back on your iPhone.