I would still try to use the reservations you have. during spring break we showed up 45 mins late to our reservations. I think the disney bus driver went a long route because when we left the beach club we had an hour before out reservations. I remember when we were waiting in the check in line there were people in front of us getting a pager for 45 mins. Once we checked in we were able to get in the waiting line. we had maybe a 5 min wait.
Or
You can call the trex resturant yourself and make a different reservations!