If they have no conference rooms left, then that is something that cannot be changed.
However, if you want to be an exhibitor, you can still book a room as long as the hotel isn't full. You just won't get the special $129.00 rate 'cause the May 2nd deadline has passed.
That's not a big deal, really, but what IS a big deal is that they add you to the LSAF exhibitor block. You must be added to the exhibitor group list. Why that is important is exhibitors are grouped together on the same floor(s).
So call 'em back and explain all that to them. Tell 'em you'll pay whatever rate they charge for a standard room but you want to be added to the group so you can be placed on the floor(s) with the rest of the LSAF exhibitors.