I enquired a while ago about using the cartridge internals from a 1k to upgrade the 650s damper rods.
The stanchions (chrome legs) are about an inch longer from what I can tell, tge 1k lowers are different, I think the spundle is thicker.
I suspect if you use the 1k legs you would need the 1k cartridge internals, I'm not sure wether they are a straight fit to the 650 lowers however.
GW suggested that the 1k spring rate & cartridge internals were not a significant upgrade over the 650 damper rod setup.
It might be easiest buying a couple of new chrome stanchions for the 650, new 'uns are just over £100 each as I recall