I've done this at least 3 times on different W124's. There are 4 bolts per seat. Two in the front and two in the rear. The rears are slightly tough to get to. Simply a matter of having the correct tools.

First tilt the seat cushion up down in front up in rear using the control on the door. Then move the seat as far forward as it will go. Now look for the outboard bolt head in the seat track. If I remember correctly it's either 13MM of 15 MM, the same head size as the front bolts. It think I remember using a 1/4 swivel and an extension to get at the outboard bolts. I am vaguely remembering removal of the plastic trim piece on the outboard side of each seat.

After you get the rears out, then move the seat all the way to the rear and remove the two front bolts. Then you will need to unplug the plugs under the seat. Kinda of awkward to reach them.

Be very careful when removing the seat from the car. Very heavy. You can easily damage the leather trim that runs up the center pillar.

Steve