A booking is made for one specific aircraft, so pick the registration you will really load. The rule behind it is strict: the type booked must be the type flown. Fly another type and the report is invalid, even if everything else about the flight was perfect.
Changed your mind? Rebook. Swapping the aircraft after booking and filing against the old one does not work.
Source: OM-B A32X, 7.1 and 7.5