As others have mentioned, you would have to sell all 160 points,
DVC won't split up a contract.
However, there are ways to "beat the system" in this case. You could purchase a 80 point add-on (though you might have to waitlist for awhile if you wanted it to be at BCV). Then you could sell your 160 point contract and keep the 80 pointer. If you had to finance the 80 points, you could easily sell the 160 pointer and then pay off the 80 point contract in a very short time.
Just an idea.