The register password is retrieved by opening the MaxiSys App , click settings, then click About. You can view details about the unit including the serial number and register password.

If you have no access to the old email, which should not have any affect on the updates or your autel login, and you want to move the device you have to contact autel at support@autel.com or support@auteltech.com as they can de-register the device from the old email/account.

So, if you ave access with the old ID to autel website you can release the device from the account yourself and re-register to a new account.
If not , you need autel support assistance.