What's your motivation for upgrading to an AP immediately (versus upgrading to a hopper and then upgrading that to an AP)? Just to save time and not have to double-upgrade? Do you not need/use Magic Morning? Or are you in Southern California and have monthly payment issues that I (as a non-local) don't quite understand?
I know that even if you are able to upgrade at the ticket window, then if you haven't had an AP before, you will still need to go to the AP processing center for a photo. And I know that if you go to the AP processing center on a blockout date and upgrade a valid pass to an AP, they don't throw you out of the park--your admission IS valid for that particular blockout date only.
I do NOT know if you upgrade a ticket BEFORE ENTERING THE PARK, if you can still get in, or if it might cause problems.
So even if you don't want/need a Magic Morning, then IF you will need to go to the AP Processing Center anyway, I don't see why you don't just go in on Saturday with your GAD ticket and upgrade that once you're in
Disneyland. I don't think it will end up taking much (and maybe not any) more time than upgrading at the booths (again, if you even can do that). It should also help get you in the park quicker on Saturday--if you're going Saturday near park opening, that's pretty valuable, IMO.
Or if you can use Magic Morning on Saturday or even Sunday, I would personally buy a 3-day hopper and then upgrade that to an AP sometime after your MM.
Have a great trip!