how to switch back to old apple ID
I have an apple ID with a @live.com email address. I switched phones and but couldn't remember the password so to speed up the process of getting my new phone set up, I created a new apple ID with a @gmail.com email address. I'm trying to switch back to my old @live.com ID but it says that it is not an available email address when I try to change it in settings on the new phone. How can I go back to the old id on my phone so that I don't lose all my purchases?
iPhone 15