Well, it is possible to move email addresses between AppleIDs. I had 2 AppleIDs, 1 for purchases/messages/facetime and one for everyth’ng else (iCloud). I didn’t want to delete either account (losing purchases or iCloud data) but I wanted the email address that was the purchasing appleid as a “also reachable” email address on my “primary” iCloud account so that I could use things like messages in the cloud and apple pay via messages. After pulling my hair out at several points, this is how I accomplished it.

1. log in to appleid.apple.com with the purchasing apple id.
2. use the “rename appleid” option to pick another valid email address. Having a vanity domain makes this simple :). Verify it.
3. ON ALL DEVICES log out of the old purchasing/messaging apple id. This is where I stumbled as I have more devices signed in than I realized and thought I’d caught all of them.
4. log in to appleid.apple.com with the primary icloud account
5. add an “reachable at” email address with the old email address and verify it.
6. log into app store on all the devices with the renamed purchasing appleid.
7. log into messages and FaceTime on all the devices with the primary iCloud account.
8. Turn on messages in the cloud on all the devices.
9. Blog it.