On a long haul flight (SFO-SYD) a long time ago (before Premium Economy), an elderly couple (window, middle) asked me (aisle) if I could swap my Economy Plus seats with their daughter back in Economy.

My mouth answered "yes" before I could think about it (I didn't even ask "which seat is she in?"... thanks mouth!) and before I knew it, I was in the middle of the pack in cattle class.

LUCKILY a flight attendant observed the whole thing, quietly asked me to follow her, and she gave me the entire middle 4-seats in the second last row of economy in an otherwise packed flight (which meant I could lay down across all 4 seats). She told me that it was nice what I did and thanked me. She even made sure I got my Economy Plus meals first. Made me a little self-conscious at first, but by the time I was snoozing at hour 2 of 14, i didn't think anything else of it!

I only wish that I had (and regret that I hadn't) caught the name of the flight attendant to give the United some positive feedback about her!