Depends on the flight. For a red-eye long haul flight, I would change into the pyjamas especially in F since the cabin is not that busy. Chances of airplane accident are just too low that I would not consider that. On J cabins, again it depends on how busy the flight is and if it is a red-eye flight. But I would certainly change into something comfortable onboard, like trainers and long sleeve T-shirt even if there is no pyjamas. I mean even in cold weather, most passengers in premium classes hand over their winter coats/jackets to the crew for storage, so even in emergencies they are unlikely to be returned one by one.