I was on a pmCO 737 operated by pmCO crew a couple weeks ago where someone in the seat across from me tried to use an expired United voucher. One FA asked the other, "do we accept expired beverage vouchers from United?" The other FA contemplated for a minute and said, "No - I don't think we do. Sorry." I got the impression that if it was a CO voucher they would have taken it. On a pmUA crew flight, I have seen them take the expired vouchers without question. So, just beware that YMMV based on crew and confusion. I'm waiting to use some of my ten year old, old-style UA drink vouchers with the 1K logo on a CO crew flight just to see them scratch their heads.