If you borrowed, make sure you are going to use them.
For example, say you have Dec. UY, 100 points contract.
If a reservation needs 150 points, than you need to borrow 50 points from Dec. 2020 UY.
If you do that, and than cancel, those 50 points now stay in the 2019 UY, and the points will expire at the end of Nov 30th, 2020.
You can't un-borrow borrowed points. So, don't borrow unless you are certain you are going to use the points.
ETA: If you already borrowed (those points already now in 2019 UY), than what you can do is banked your 2019 points into 2020 UY, so that the old 2019 points starts Dec. 1st , 2020 and expires on Nov. 30th, 2021, so in a way you can kinda undo the borrowing, in a round about way. But than you don't have any flexibility in 2020 like you would normally have if the points were originally 2020 UY points.
Just remember that banking and borrowing are final transactions and can not be undone. Since you are traveling late in your Dec UY (A Sept trip), should you have to cancel, you don't have much time left, and must use up those borrowed points by Nov. 2020, so there are lots of risk there.
Hope that helps.
Great3