I don't think you want to do that! Otherwise all your content on the 'old' 7 year A-ID will be invalid for future download of said content in future, when signed-in with a 'new' A-ID email account.
One thing you can always do, is go to
https://appleid.apple.com and change your Alternate Email Address to the email address you currently use, for receipts and all Apple comms to get sent to that email address, but still sign-in with your mac.com (or me/icloud) email as your A-ID for purchasing.
Remember that mac.com/me.com/icloud.com email addresses are all interchangeably using the sme inbox and background functions on your account (provided you were using the previous service before they went on to the next one – which in your case you clearly were as a mac.com email user), so you could use the later iCloud.com if you wanted. Or you could ignore the email function of the A-ID, and just continue using it as the sign-in for A-ID for various Apple digital stores purchases.
For the foreseeable future all mac/me/icloud email address A-ID's
cannot be changed to a new email address either. And that's highly unlikely to ever change.