If you are going to install prefabricated seat cover kits, then there is no problem. Once the old covers are off and the foam cushions replaced, the trick is to slide the new covers over without bunching up the foam.
Place a large plastic trash bag over the seat back, from the bottom. Now the seat covers will slide onto the back very easily. Once in place, pull the trash bag out from the bottom. Repeat the procedure for the seat cushions, beginning from the back.