When I first bought, the easiest way for me to remember that any trip can be "paid" for with banked points from the UY prior to travel, points from the UY of travel, and borrowed points from the UY after travel.
Doesn't matter when you call and book, just when you will go. Since a Dec 2015 trip happens in an Oct 2015 UY, then banked 2014 points, 2015 points, and borrowed 2016 points are all available to use when booking the trip (as you figured out!)
Just be careful, though, with that Dec reservation and taking chances at 7 months. That is an extremely busy time and once you cancel and go to re-book, if the new room is gone in those few minutes, you may not be able to get the original one back.
In this case, I might call MS at 7 months and have them do it if there is availability. I know that some say they can't hold things when making the switch, but I have had CM's tell me they could and when they couldn't the CM was very clear about it. I always asked up front if there was a risk as well.
Just something to think about!
Good luck!