I think you would be safe fitting leather seats from another FL - it would be almost impossible for them to ascertain if they were fitted by you at a later date or fitted at the factory as an upgrade. It's not something that they would be interested in anyway as it doesn't change the risk profile, but as others have said it has been shown that sometimes they will capitalise on any slight modification as a reason not to pay out. In this case though the only change is the seat covering material. How many of us have fitted roof bars, light guards, side steps, alloys etc?
As for gstuart's situation - changing the seating capacity of a vehicle is a major modification in the eyes of insurance companies and the DVLA. Although it's a minor mod in a FL commercial which was designed as a 5 seater anyway. Had he not had the V5 amended he would have been committing an offence carrying more than 2 passengers (exceeding stated seat capacity).