Did a foam swap this weekend. I got adonor seat from a '92 200 to use for my '89 200.
Here's some findings that may interest you if you are planning on doing this job.

Bottom Seat area: 1. Spring/Wire assembly is slightly different, The '89 has a thicker wire running down the center wher ethe '92 has none.
2. Cushion of '92 is a one piece unit, the '89 is a two piece, the front area where your knees are is a seperate piece. Volvo Probably thought that this section will wear out and made it easily replacable.
3. Seat warmer. The '89 has a hard plastic assembly with insulated wire running up and down. The '92 has this spongy material with very fine wire running in it. Personally, I think Volvo went cheap on the '92 heater.

BACKREST AREA: 1. Foam looked identical
2. The steel frame is slightly different. The Bars that give the curve in the seat are shaped different.
3. On the '92 there were additional springs supporting the foam (six total) on the '89 there were only 2 springs.
4. The '89 had separate Height off the floor adjustments. one to lift your knees up and one to bring your whole Backside up. The '92 didn't have either. The '89 is much nicer. The hole pattern to mount to the car is identical. If you are short, look for an '89 frame.
5. The seat heater was the same scenario as the Bottom. Hard plastic vs spongy material.

I took about 30 pictures of work in progress and modifications I made to really support the foam. So the job probably took twice as long. I plan on making up 3 documents, Bottom seat dissassembly, Backrest dissassembly and Modifications and assembly. But in the mean time, If you want to see some Pics, E-mail me.
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'75 Jeep CJ5 345Hp ChevyPwr and two motorcycles: it wasn't Volvos safety , it was Longevity that sold me