I'm trying to make sure I understand this correctly. So, if you start paying on the monthly plan now (and Disney will take the first monthly installment out of your account within a couple of days, even after whatever down payment was paid), to lock in the price before it goes up, the AP still won't be activated until you use it in December? Interesting! You learn something new every day!
I've done the payment plan a few times but have never activated my AP months later. In other words, when I first signed up for the plan it was activated right away and I was charged right away. Ever since then it has just been a renewal under the same terms, basically. So I've never had to deal with paying now and activating the AP much later on the payment plan. It's always been a pay now/activate now thing for me.
I knew that AP's could be activated later if someone bought a voucher now, but I didn't know it worked the same way with the California resident payment plan! Cool!

That's good to know!