OK, this is my exeprience only.

I'm one of those that fly steerage for long haul flights and it never occurred to me to change clothes for overnight flights. I can't imagine why I'd do it, regardless of what class I was flying. I always fly in comfortable clothing. Part of my go to ensemble for overnight long haul flights is a pair of trousers with a bit of stretch (made by Calvin Klein, these are nearly as comfortable as leggings...waistband, zip and all). I can contort myself into whatever random position for sleeping and the trousers are perfectly comfy yet still hold their shape when I'm up and moving. I pair that with a blazer and either loose button down or vest. Business to casual by removing the blazer (if I'm warm enough).