My Experiences
I used to always fly in a business suit even one one cabin planes, because it meant I could bring one more suit with me than if I crammed it into my suitcase. These days, I've learned to wear practical clothing regardless of where I'm sitting. I don't think I would I wear my dressiest black suit unless traveling for church business.
When I fly at the front, I don't usually dress up in the traditional sense. I feel that slacks, a sweater, a casual leather or cotton coat, and sometimes a shirt which can be worn with a tie at a later point are enough.
I look at it like this. Can the coat and shoes be worn for other purposes? If there was something I'm wearing which could not be used elsewhere, I won't wear it.
The only exception is when traveling purely for pleasure. I will not seek an upgrade as I don't have status or pay for the premium seat. In that case, I typically wear what I'd like to wear the next day on the ground.
I've never flown TPAC or TATL so the longest I've ever been on a plane isn't that far. I've met people who will wear say, yoga pants and a tee, but it isn't my style.
Chuck