Can't change purchased itunes videos to have media type other than Home Video

I have a number of files on my new Mac mini, imported into iTunes (12) from an external drive that had been serving as storage for my previous iTunes library on an old laptop that is now defunct. For some reason, most of these video files got changed to have their media type be “home video”. I have changed the media kind metadata on the files that were DVD rips, but I cannot change the media kind for the files that are iTunes purchases. These identify (in the File tab) as “Protected MPEG-4 video file” and play fine.

I can also see these files through Home Sharing on iPads and my Apple TV.

Nonetheless, I can’t change the media kind to “Film” or “TV programme” as appropriate, which is very annoying. Is there any trick I might use to fix this?