Upgrading to the Premier Passport should be no different than upgrading to a typical annual pass, and they can probably do it right at the will call window.
I suppose there is an outside possibility that it needs to be done at Guest Relations (due to the nature of the PP), in which case you'd just redeem for the 1-day ticket, go inside the park, and upgrade inside. But as far as I know it is available at the ticket booths like any other park. Cheshire Figment I'm sure can confirm that.
It can be done at ANY of the parks for sure though.