Ganymed (Mp3tag developer) is currently away from his computer, he'll come back to it in about 2 days

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You can rename a batch of your files using "ren *.mp+ *.mpc" in a command line window inside the folder with the mp+ files. (or "ren *.mp+ E:somefolder*.mp+ *.mpc")
Or you could use some program that specializes in batch renaming, like
renamefiles.