What happens if my apple id email changes? what can i do?

I have changed my Apple ID (which is just an email address) several times and it is quite easy if you follow these instructions:

https://support.apple.com/en-us/HT202667

Make sure you sign out of your iCloud/Apple ID account with the device first.

The "name" you use for your Apple ID "username" is typically an email account name. However, it is not at all tied to that account and you should be able to continue to use it as your Apple ID even if the Comcast account is no longer active. However, you do have the option to change a third-party email address to an iCloud one.

Ref:

• About your Apple ID email addresses - Apple Support
• How to Change a Third Party Apple ID Email to iCloud Email - OSXDaily