Getting changed into PJs in an airplane loo is fine - I take hotel slippers with me for the flight, and because of their slip on / off nature, can manouvre myself into PJs without ever standing on the floor, or letting my PJs come into contact with the floor. I change into PJs for most long haul flights I take, whichever class I am in (I bring my own), and I *shock horror* don't wear anything underneath them!