Hi Wonky and Winifred - thanks, I thought about this, but would have had to upgrade two deposits as the change triggers a full deposit. Decided to leave for now, but may come back to this idea later.
The on board person told me you can sometimes split into two bookings, which together have a higher value, so I’m thinking about this too. However we would end up with even more cruises then! I think a lesson to the wise is to book the longest dates Cruise at the lowest value as a placeholder when on board, and then bring forward.
thanks again