When your iTunes receipt email bounces for the first time, iTunes will automatically ask you to update your AppleID.
The fact that AppleID uses the email format convention for the “name” field, is only that, a format convention.
However, why not go ahead and change it right away at:
https://myinfo.apple.com/cgi-bin/WebObjects/MyInfo
Here you may change your AppleID to any email address you like. This will change your login across all Apple applications and services you may use.
One curiosity though, when you authorize a different computer to play content from your purchased library, you may have to log in with your “old” email address in the future.