As long as the days booked fall within the prior UY(08) then you can borrow the 09 points....So....
If you have an Oct UY and you wanted to book Sept 20-27, 2008 and borrow from your 09 points, you would be out of luck since the days booked do not fall within the prior UY (08) which begins 9/30/08.
This is where banking and borrowing come into play...you could bank your 07 points into 08 and borrow your 09 points into 08 and have triple the points to use in UY 08.