In reply to jamieevans:
Currently modifying an old Alpkit Heiko by cutting off the arms and moving the insulation from the arms to the core. So it's now more of a "belay gilet."
As mentioned above the primaloft is formed in sheets (removing it from the arms by unpicking all the stitching holding it in place was tedious to say the least!) Once removed it's quite easy to work with - I cut open the liner at the hem, slid the sheet of primaloft in and tacked it in place.
The Xenon differs from the Heiko in that the primaloft is stitched to the inner whereas the in the Heiko it's stitched to the outer (you can see the external stitching in photos) I would assume this helps with the weather resistance of the Xenon and it also means any stitching you do will be hidden - definitely a good thing if your sewing is as untidy as mine!
I would say that its MUCH easier not to remove the old primaloft, instead just sew your new sheets directly on top. Also the hardest part will probably be replacing the insulation in the sleeves - I suspect the easiest way would be to cut the liner around the armpit and try to fiddle the new sheet of primaloft in there before tacking it in.
I have some photos of what I did, can send them to you if that would be useful. (Managed to do it all by hand, sewing machine not required but would definitely speed things up)
Let us know how it goes.