I have been rebooked on SWA by HP before but they gave us a check and we had to go to the ticket counter and buy a ticket. SW wouldn't just take the endorsed HP ticket like othe carriers will. If they(HP) called SW and said we need X seats SW may have told them no becuase the flight was full or close to full, but you as an individual can go buy a ticket becuase they do oversell the flight (they just won't do that in bulk from another airline I assume).