Just had a very refaced chat with a lovely ? Russian in apple support. I am an idiot, it's clear, everyone probably knows this...
Say your apple id is [email protected]
And your default email is say [email protected] and you change it to [email protected]
Then....... Your apple id also changes to [email protected] and trying to log in to [email protected] will lead to tears and hair tearing etc
Why did I it realise that?