If you cancel the FP+ reservations on the days you are dropping before you drop them, I would think there won't be an issue.
You probably know this but: You need valid admission in hand before making FP+ reservations. The only way to do that such that you can easily downgrade the number of days later is by buying a package. If you drop the cost of your package after your final payment is made (must be at least 30 days before check-in) there are fees that amount to $65. So, you've got a narrow window to make changes. If you don't want to be tied to a package and its more restrictive rules, you can buy a shorter ticket and upgrade it from home if you can go longer as soon as you know that. This way you can't make the FP+ for the "extra" days until you add them, but if you are making the hard-to-get FP+ for the days you are sure of anyway, this is no big deal.
I also seem to recall that dropping days on resort reservations is no longer trivial; you'd have to be able to re-book as a new reservation, which isn't always possible. Maybe one of our Disney TAs can advise.
So, if it were me, and assuming there isn't also some big package discount you are taking advantage of, I would book two room-only reservations one for the longer stay the other for the shorter stay. You need different lead guests on these two, IIRC, but as long as you have two or more adults going, that should be fine. Then I'd buy tickets for the shorter one. Once I knew the length, I'd cancel the room I didn't need, and extend the tickets if I needed to.