I am not sure it is an ironclad rule that a log in is written to the device. I have checked the ttgo.bif file and a log in name is not always written to it. If you logged in to Home, Home will sometimes write the email to a tthome.bif file. Sometimes it writes it to the ttgo file, but not always. I hooked up a 540 XXL TM today that I had registered last month for the lifetime map updates but did not have the time to update until today. I checked the ttgo file before I signed in to home (because I had forgotten the email) and no email addy was written to it. I use an older version of home, 2.7. But the minute I booted Home, it auto displayed the registration email. So it may store it on the PC, or it may store it elsewhere on the GPS, but it was not in ttgo and there was no tthome file. I later found the email, because I always write them in pencil on the box and use the same password on every unit I have ever registered, just to unclutter things.
I am not sure if you're asking to change the email on the map account or just change the email to get regular updates. TT customer service will change the email addy on a map account if you have the email and password and the map voucher code. At least they did for me.
