Just hit 'use unregistered account' - logging in as a 'registered account' but leaving the email address blank is logging in the same trial account on the server, but your client doesn't know it's trial. I'll fix that behavior for the next app submission, but it doesn't cause any incorrect behavior as far as the server is concerned.

If you want to be certain to keep your characters across reinstalls / firmware updates, you'll need to use a registered account.