Yes on the back rest I am only redoing the fabric on the inserts.
But I am also redoing the fabric on the center of the lower seat.
The seats came from a car I purchased and parted out in 2007, these models of cars have no cup holders, it would appear who ever owned this car used the passenger seat as a coffee cup holder.
This is after I ran it through the wash on ultra delicate and hung it to dry. After sitting around for 15years they where covered in dust and dirt.
I will clean them thoroughly with spot shot carpet cleaner and run them through a delicate rinse cycle before I start working on the upholstery. There is only one visible stitch on each section I want to replace all the rest is stitched from the back side.
Here is the backside of the lower cushion cover.
Washing got a lot of the coffee stain off the white mesh covering the foam but not out of the vinyl/white cloth peaces with the piping and hog ring holes.
My main reason for wanting to keep the old fabric under the new is the way the mesh and foam is stitched to it and the extra hog ring strips that drop down into the lower cushion, what a pain they where to remove/cut.
There are 11 hog rings holding the seat cover into the cushion, I didn't expect to find this after removing the +-30 hog rings holding the upholstery to the bottom side of the seat, I don't pull upholstery off seats but have watched upholstery being done on hundreds of car shows and online threads, never seen covers attached to the foam this way.
If I keep the original fabric under the new fabric, all the mesh, padding and hog ring strips and piping would stay stitched to the old upholstery, I would only have to take out the straight stitches where the cover fabric is stitched together, add in both pieces of the new fabric and sew along the same stitch lines on the old fabric.
If I ditch the old fabric and replace it I would need a sewing attachment to stitch the pipping to the new back rest fabric and do about three times more sewing to the lower seat cover. And increase my chances of not getting everything in the right spot and have things not fit by about 10X. I see undoing the stitches then stitching it back together exactly the way it was as a challenge. Heck getting the plastic headrest guides out without damaging them and the 11 hog rings in the seat cushion where a challenge.
And I have no intentions of attempting to install new hog rings in the lower cushion grooves and align the open hog ring, put the upholstery on the hog ring, insert pliers and close the hog ring in that small groove. I will use industrial zip ties (the kind with metal retainers), just tighten the zip tie until it's the size the hog ring was and cut off the zip tie tale. When cutting out the hog rings I wondered how on earth am I going to put this back together, it looks like when they assemble the seats they installed the hog rings on the upholstery and put it over the cushion foam, then they take a metal rod dipped in glue and insert in through the backside of the foam and thread it through the hog rings, then they glue the foam to the metal base of the seat covering the holes in the foam made by inserting the rods.
On a side note:
I don't know what the upholstery industry uses to clean stains but Spot Shot hand pump spray carpet cleaner is amazing stuff over the years I have cleaned automotive upholstery and carpets to look like new using spot shot, gotten cat spray smell out of seats using spot shot and a pressure washer and got black/green axle grease out of a white suede seats in a brand new Mercedes one of the neighboring shops was working on, he sat in axle grease by accident then moved the car. He was going to buy a new seat (the seat costs more than I ever paid for a car in my life) and it would take more than 1 week to get, he was freaking out about how he was going to tell the lady who just bought the 6 digit price tag 2 door, 550hp luxury sports car as her retirement gift and brought it to him for a few custom modifications that he ruined her seat and can't get a replacement for a while, I asked him "did you try spot shot", took less than 10 minutes to clean the seat back to new, not a trace of the stain in the suede.
You probably all use something better and know about spot shot carpet cleaner but on the offset chance some people don't.