Unfortunately, once the bookings of B-Ticket are confirmed they cannot be changed or refunded, as stated in the Rules. The rules of the ticketing service are defined by the museum, and they may be found in the ‘Terms and Conditions of Service’ section, accepted by the client at the time of purchase.
In case of early or delayed arrival (on the same day of the reservation) the ticket you purchased will still be handed over, but you will lose your priority of entry and will have to do the queue at the entrance.
The only exceptions is Michelangelo's Secret Room, since the visit is made in limited groups for each of the time slots.
By arriving at an earlier or later time than the one of reservation you won't be able to visit the institutions.