What happens when you try to change the email address? Error message(s)?
Note the restrictions in the article.
Apple ID Name Change. *
Make sure you turn of Find My Phone before signing out of the old ID. Once signed out and Find My Phone is off, then sign in with the new ID, then turn Find My Phone back on.
What to do after you change your Apple ID or password - Apple Support * I suggest doing this before you change your Apple ID or password.
Have you logged into your Apple ID to see if the address shows under Reachable At, as a Rescue email address, or as a Notification email address? If it is listed, you can remove/replace the email address.
Apple ID - Manage
Apple hasn’t published any guidance for using an address again. Normally, if your e-mail address was associated with an ID in any way, for example as a Notification email address or a Rescue e-mail address, then it can't be reused as an Apple ID email address for approximately one year. I have also seen posts that stated Apple told them there was a 30 day waiting period before the address could be used again. You will need to get another e-mail address from somewhere like AOL or Yahoo to use as your Apple ID or you can try waiting 30 days to see if that works.