I have a 2004 with out the seat bottom that powers forward and back to extend or retract the support on your legs.

I have access to a 2005 Vanden Plas with this feature.

My interior is black and the VDP's is cream.

Is it possible to swap out the seat lower frames? Would it work?

Any thoughts?

 

Off the top of my head, I can't imagine it would be worthwhile to make it work. I doubt your seat bottoms extend enough match the length required for the VDP frame....
I could be wrong though.

Certainly don't blame you for wanting to try, that feature is quite nice, I have mine adjusted just right and my passengers love being able to adjust it.

 

Similar interest here. Vanilla 2005 XJ8-L, 'ordinary' power seats, as my own start-point.

Lady Wife just under 5' tall has always had to utilize a bolster to not have the front edge of the seat hit ahead of her knee joint.

Swapping the entire seat, not cobbling together disparate upper/lower halves, is the sanest way forward.

Should fit at the floor, as -'AFAIK' - they were available options, even on non-VDP, non-LWB. Would not be surprised if the 'memory' NVRAM already has space to store the extra setting as well. Not a hardship even if not.

Once put Chrysler swivel-bucket seats out of an Imperial Crown Coupe into a vanilla Plymouth Fury III S/W. Direct bolt-ins, so wouldn't expect much to be needed at the floor. Automakers try hard to reduce their need of alterations or extra inventory in that regard.

If not, the machine work to fab adapters is white-bread - basically saw and drillpress work.

 

The parts system shows different part numbers for the 12 and 16 way seats for the "occupant compartment harness" so you may have to change more than just the seats.

My SWB X350 had the 16 way seats so you should be able to make something fit.

 

If that alternative 'harness' does not involved a tunnel-side anchor and latch that is easier to reach when seat back is further forward vis the seat platform, it certainly should do.

The 'vanilla' item is already a bit unhandy as-had for an oldster with limited flexibility. Which stands out, BTW, as virtually all-else is very well-placed for ease of use.