Before you take apart the seats entirely, try swapping just the sliders (4 12mm nuts) and see if the seats fit with the knobs nearest the inside. I just went through the process of adding the height adjuster to a passengers side seat (salvaging it from a drivers side seat). I found that the drivers side frame & sliders didn't fit quite right on the passengers side, so I ended up changing the sliders after I had already changed the entire frame. The nuts holding the seat to the floor are 13mm, the drivers side frame nuts were 13mm, the passengers side frame nuts were 12mm, and the slider nuts were 12mm. The 12mm/13mm Volvo wrench that's part of the tool kit was very helpful to get in where my 13mm deep socket wouldn't fit.

Good luck.