Heavens! I always change into one of my many BA/MH/VS PJ bottoms and t-shirt (I have only every like the CX PJ top) the moment I board the plane. Have done so for years. I make a bee line for the toilets after dumping my bags on the seat; while the plane is on the ground and no moving.
I change back about 30-45 mins before landing.
I often see other guys in shorts and bare feet. Cannot bring myself to do that.
The other half rarely changes to fly long haul. To each his own.