MacRumors Forum:

For a very long time, it has not been possible to change your Apple ID email address if it was an iCloud email address, say from iCloud to Gmail.

I’ve just discovered it’s now possible, you can change your email to any email address you want – and back again. I note Apple’s Support pages have also quietly removed any reference to this (now removed) limitation.

This is one of those things that I’d normally try to test/verify before I post but, for obvious reasons, no easy way for me to do that. But I do recall this limitation, and other folks have stepped up to say they were able to make this change.