I recently booked 3 weeks on the PG for next year - we are also booked on the Navigator this Fall. When making the first 2 week booking on PG, I inquired about on board booking on the Navigator to apply to PG. "No problem" - just give the booking number and it will be applied. Two days later when adding a 3rd week to the PG trip, I raised the same question - different answer - it is treated on a "case by case" basis. What are the cases? I ask. No answer. Ngaire spells out the cases in her reply on this thread. They seem reasonable. Question? Same category cabin has to be available. In this case, it can't be - I've got it! Only one owners suite on the boat! It seems like the onboard booking credit is a tiger by the tail - a lot of trouble but difficult to get rid of.