Changing my primary iCloud email address

I am helping a friend with some longstanding email issues. Situation:

He has a gmail email address that is his iCloud userid. He has a primary iCloud email address that is very cryptic and not used currently. He wants to change this iCloud email address to something more useful and memorable to others (eg. firstnamelastname@icloud.com), and then change his iCloud userid to this new iCloud email address.

First issue:

I have tried changing his userid to a different email address, and while that seems pretty straightforward, iCloud doesn't seem to permit a userid with their own iCloud.com email domain?

Second issue:

I cannot find a way to change the primary email address. I have also tried creating an alias, thinking that possibly the alias could be used as the userid, but the option to add an alias does not show up for him (he does not currently have any email aliases), and according to support notes I have found an alias is not permitted to be a user id.

Any help would be appreciated.