They actually get the Benacquisto from the state (not from UF) and if they get that, they do not get BF.
You can ‘save’ your prepaid for grad school if you don’t want to use for undergrad if you want. You just have to let your school’s finance department know if you don’t want to use it and they will apply just the BF and bill you (or use scholarships) for the remaining fees.