Originally Posted by Empress
Keep in mind that the actual PNR and ticket are two seperate things. As long as you have a paper ticket, they could build a new PNR & accept the ticket coupon (Of course if the paper ticket is showing as a restricted ticket and different date, you can be sure they will look into it). Same with an E ticket; they just need to build a new PNR, associated the ET with the PNR and you're good to go.
If the flight is completely sold out (or at least if the booking code/fare class is completely sold out?), then they can not build a new PNR for that flight, even if the ticket is showing that flight?