Yes, you can cut it down to 5 nights without a penalty if you do it more than 45 days in advance. If you make that change less than 45 days in advance, then there is a $50 fee.
If you are asking whether they will give you credit for this year's one-day pass toward buying next year's 10-day pass as part of the FD package, I believe the answer is no. But I think you could book only a one-day pass for next year, and then upgrade that pass to a 10-day pass when you get there; or you could save next years's one-day pass for something else and upgrade this year's one-day pass to a 10-day pass next year. Confusing, huh?
Another thing: I think you can only upgrade unused passes at the parks or DTD guest services. I tried to do it at our resort, but they said no.