As long as you have current UY points, they won't let you buy the one time use points, even if you are at the 7 month mark--at least they would not let me do that.
They will only let you buy them if you are in a borrowing situation and want to avoid doing that.
Since, at this point, you already have your August trip booked, you can't just ask to buy the points and have them put in to that trip to free up points for the December trip.
I think to make it work, what you would have to do is cancel a night from your August trip first, book the December trip, and then quickly re-book the August night again.
I did something similar when I was booking my December trip. I knew for sure my August trip is on but always a chance my December trip for DH and DD might not so, since I would need to borrow, I wanted the borrowed points set up with the August trip and not December.
I called, canceled one of the rooms for our August trip--SV at BLT--after finding out there were LV rooms available and enough of them that I would at least be able to book that---and it freed up enough points for the December trip.
I then re-booked BLT for August using borrowed points--I could have bought points if I had wanted to at that point but did not--luckily, in the 10 minutes that I cancelled and re-booked, the SV room I had just released was still there and I got it back!!!
Hope I didn't confuse you but I think that is the only way it will work.
Good luck!