As I understand, W126s and W124s have different floor anchors for the seats, so it may not be a simple swap.
What I know for a fact is that manual and electric seats do have DIFFERENT floor anchors (even in the same series), so to change from a manual seat into a fully powered one requires changing the whole floor plan at a very high cost $$$$.
CL 500, 1998
S 500 L, 1998
E 320 T, 1995