When you book a trip, they determine which points you can use based on time of travel and not when you call (or the 11/7 month window for any reservation would not really work).
For example, I have a June UY. If I want to travel in August of my June 2011 UY, then I can used "banked" points from 2010, 2011 UY points, and borrowed 2012 UY points.
I could acutally make this reservation in Septmeber 2010 (still my 2010 UY), but since I am traveling in my 2011 UY, MS will borrow the 2012 UY points if I need them.
So, if you are traveling in the UY prior to the UY of the borrowed points, you are fine.