Hello retiredcoachTC,
Email addresses can only be associated with one Apple Account. For example, I cannot have my Gmail address in both my primary and secondary Apple Accounts. If it tells you that the new address cannot be added to the account, you might have an Apple Account made with it already. You can visit https://iforgot.apple.com, tap reset password, and enter your new email address. If an account exists that is associated with it, it will send you an email to reset that account password.
Note that you must first add a new email address to the account, assign it as the primary address, and then remove the old address.
i hope this helps!
Sincerely,
Captain