Change Apple ID and iCloud email

I don't think you can without creating a second account and using the email address for that as an Apple ID for the first account.

You cannot change the identifier used for a primary iCloud mail account. You can't change your a to a b, you would have to create a second Apple account with b as the primary iCloud mail address which after 30 days you could use as the Apple ID for the first account. Probably more trouble than it is worth. You can only set up aliases as alternatives, but you can't use an alias as an Apple ID.

Using the two account method would mean you would still be using the old account as a primary account (so not transfer of anything necessary). Setting up a second one would just be for getting an email address/Apple ID you can live with. You are setting up the second account so that after 30 days you can set up an iCloud Mail account with your B address. You then change the Apple ID email address on account A to that of account B. It is a bad idea anyway to use an iCloud mail address for the Apple ID of the same account. Should you need help, Apple will be sending emails to the very place to which you cannot get access.

I won't go into this but you can transfer some data between accounts but it is somewhat involved and you cannot transfer everything.

As I said previously, this is going to get finicky and I am not clear why you don't just live with things as they are.