You could call and ask. There may be seats that are blocked to you that an agent would be willing to open up because of the equipment swap.
DL currently has many different kinds of 757's, because DL and NWA both flew them and each had several different configurations. They are now starting a process to try to streamline them but this will take a while.
Also, when the 24 hour mark hits before you flight, log in right away, as more seats can open up at this time.