You could be like the attractive 20-something lady on my LAX-HKG flight and just change at your seat while barely trying to keep the duvet over you...
Much less was left to the imagination than you might think
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Seriously, though--I always change onboard. I enjoy dressing up for flights, though, and I don't see it as preposterous if people show up in decent-looking PJs for late-night departures.