You can't change a registration once it is done. The only way to do it is to either install the software on a different computer, or format your hard drive, reinstall windows and all other programs (you don't want this option)
Actually I'm getting ready to do the second option, nuke the C: drive. Windows has slowed down enough over the last year where it's time to clean it up, so I'll be doing a fresh install, and moving to Windows 8. Does that mean I can use a new email address when I enter my authorization code?