Moving a primary domain in Google Suite

Hi folks... I have both "Domain A" and "Domain B" both with their own Google Suite accounts "Account A" and "Account B".

What I want to do is to make "Domain A" an alias for "Domain B".

Initial status:

Account A/Domain A

Account B/Domain B

End status:

Account A/

Account B/Domain B, Domain A

A "Mailbox X" exists in each domain so ideally contents would be merged, if not then I can always copy to an Outlook PST file and copy back after moving domains.

I could of course just remove delete Domain A and add it to Account B but I am not sure if this can be done - don't want to delete it, finding out it can't be added to another account and then not being able to reinstate.

Anyone done that?