Also from NorCal and an AP holder for several years including currently. Also usually tied to a standard school schedule because I travel with my mom who is a high school teacher. We get Sig+ passes because of her schedule to allow us to go during her winter break when all lower tier passes are blocked.
Disclaimer: This information could change once they reopen/announce more information. We have zero idea how they will fully handle things for sure at DLR. Things have always changed from what they first announced several weeks back.
Yes. You can upgrade a ticket you purchased (comps, special tickets, charity tickets, etc often do not count) to an AP. The AP must cost more than ticket. This is how I usually get my AP personally since they took away the renewal discount a couple years ago. This past NYE, I used a 5 day PH w/MP that I got at a discount from an authorized third party to upgrade to a Sig+. Received full face value.
Current AP holders have two options based on whether they were paid in full or monthly. Paid in full may either accept an extension for the closure period or they can ask for a partial refund. Monthly can either accept waived payments during the closure period and no extension or postponed payments and an extension.
As you could not activate a pass right now, you would not under currently known information received an extension. You can buy an AP right now (cannot upgrade a ticket to one), but it will not be activated. We also don't know for sure when DLR will open.
SDL did something interesting and, in my opinion, quite fair to accommodate SDL APs (and existing ticket holders I believe) as they are currently requiring dated, single day tickets to be purchased as part of an advanced reservation system. SDL APs can reserve days with their passes (1 day in a 7 day period; reserved say must be used, expired, or cancelled to make a new one), but their passes will still be extended for the entirety of the time between the January closure date and the end of the advanced reservation system. We do not know if this or a version of this will come to DLR. But it's one possibility we might see or see a modified version of.
Honestly? I personally would not plan to purchase an AP during this unless you wanted to lock in the price for the next year (so buying a voucher to activate when things are more settled). I wouldn't even be planning to go for awhile if I didn't already have an AP. We don't know when things will open and there's a very good chance the experience could be drastically modified for a period of time. Especially with a schedule locked in to the more crowded times I would be cautious planning to buy a ticket with the intention of upgrading to an AP.
There's just too much unknown.