And, to add, you can only borrow what you need to complete a reservation that you are making at that time.
Eveything is based on the UY you will be traveling and not the UY you are in when calling.
For example, I have a June UY. When I call in Sept 2011 to book my August 2012 trip, I will be able to borrow 2013 UY points because my trip takes place during my 2012 UY.