If you remove the backrest on the older seats you will be left with holes that I found no way to fill. I did later have to recover the seat & that made it easy to cover the holes. Removing the backrest solved my backpains almost overnight.
You would have to change the front mounting/pivot pieces. They are different.There are plastic button push in thingies, you can find at the big box home stores or even craft stores that will cover the holes nicely on the older style seat.