Not so sure of that. Last month I was on a CRJ700 in E+ and my UDU hadn't cleared at the gate. As the door closed there was still an open seat in F. Thought GA might come on board and move myself or another 1K/GS into it but nada. However, some guy got out of his seat in E+ and took his bags from the overhead and just walked up and took the F seat. Best I could make out when the FA challenged him he flashed his 1K card, the FA shrugged and let him stay! Unfortunately YYZ UA ground staff are not doing things as they should...