Now that I've had time to look.
- Go to icloud.com and login to your user account.
- In the page that opens, click on the Account Settings link.
- Near the top-left of center, click the Manage link.
- Login again on the new page.
- You should now see your account, showing your Apple ID (email address), phone number, birthday and other info. Click on the Edit button to the right of the top Account section.
- Where it shows your current email address, click the Add More link.
- By default, it asks for a new email address. Fill it in and click Continue.
You can now login to your account from either email address. One small nuisance. You can remove the address you just added, but you cannot remove the original. That may because I was still logged in from my Mac, iPad and iPhone. If I were to log out of all of those and into the new email address I added, it would then likely let me remove the old one as it wouldn't be in use.