It's easy if you have a commercial sewing machine, hog ring pliers, and the experience to lay it out, cut, sew, and fit the new upholstery. There's a good reaosn good trim shops charge so much for a reupholstery job. It's not impossible to do a decent job as an amateur, but it's going to take you a while to get it right.
I have a 1986 Toyota Tercel wagon. I recently swapped in a set of SR5 seats, so now the interior doesn't match. When I pulled them from the original car, I pulled the covers off, rebuilt the driver's seat with foam from another passenger seat, ran all the covers throught he washing machine, and reinstalled them. I have 3 complete sets of upholstery from new TRD double cab Tacoma pickups. They were removed at my dealership when we installed leather. I'm going to cut all the stitching out, and do the layout on the new pieces using one of the original SR5 covers as a pattern. When I'm done, I'll have something that'll look and wear much better than what you get from most aftermarket fabric.
Then the old Toyota should be good for the next 350k miles.