Not a complete mess. As long as your email address/ID is not already an Apple one (
@mac, @me, or @icloud) you can change it:
http://support.apple.com/kb/HT5621
The email address you chose as an Apple ID becomes a user name, more or less. You can continue to sign into your Apple account using that ID whether it is a valid email address or not. The problem comes when your forget your password or need to verify something. Unless you've set up an alternative email for those they will try to go to the same email as your ID.
Anyway, that means you can still sign into your old Apple account and do what is needed.
When you change it, I recommend changing it to an Apple email address. You don't have to use that email address if you don't want to, but it will future proof you against any future ISP changes.
Besides Apple's mail service may not be the very best available, but it's far better than the email services suppled by most ISP's, including Verizon and Comcast.
Good luck