Bill - to answer your original question, if you go to the resort front desk (staying on property) or one of the ticketing windows at the parks you CAN turn in a used pass and "upgrade" to a new one. So, if you purchase the 7 day park hopper plus and use it for one day, you could even go to the tickets windows on your way out of the park that night and replenish to last for another seven days.
Catch!!! You may want to try calling Disney. CRO/the Disney operator (407 827 7700 is the only number I know for this, they will say its Old Key West Resort, but its really the disney operator) and see if they can tell you how much value they will give you for a 7 day park hopper plus pass with one day used.
According the the Disney website a 7 day hopper plus is $319 gate price and $307 advance purchase price. This is really rough math, but if you deduct the value of one day which I'm guessing is about $50. You would be paying $319 then using it one day, trading it in for a value of $269, then paying $319 again. So in all you would be paying $369. (if my numbers are even close to correct.
Good luck with whatever you choose!