For use year it is like a normal contract. You can choose one most of the time if you like, or they will give you one. Typically it's smart to get a use year a few months ahead of the fixed week, not after the fixed week to avoid missing the banking deadline etc.
FW contracts can't change, they are set. If Disney adjusts a period to cost more points, congrats you get a discount now on the week when you use the FW. If they decrease the point cost for a period you can book normally instead of using the FW and still get the lower cost. It effectively can give you a hedge against a travel period increase on the week of your choice.
Unless you meant do the fixed week costs go up at new resorts, then yes. The FW point cost is based on the normal reservation at each hotel. So a FW for a room at the Poly Tower will usually bend more than the same FW with the same view category at RIV for example