Apple has removed the "Convert ID3 Tags" command in iTunes 12.4. I don't know why. I didn't use it much myself but many users needed to convert the metadata ID3 Tag version so MP3 files imported or converted with iTunes would be compatible with other players.
There is no AppleScript command or property for iTunes that is related to this so no purely-AppleScript workaround is possible. Interestingly, SoundJam's AppleScript dictionary had a property to read—and possibly write, don't recall now—the default ID3 Tag version. It didn't make the trip to iTunes.
[UPDATE: the Convert ID3 Tags feature has been restored in iTunes 12.4.1.]