No shocks that I'm aware of have the oil and nitrogen "mixed" there is a piston that seperates them. Resivoir's just increase the capacity. Internals as far as shim stacks / pistons / rod guides and base valves should not be any different. Positioning of those components might be. without intimate knowledge of the shocks in question I would not try to swap the resivoirs, you'll need to contact works for oil weight and amount, and nitrogen charge PSI information anyway.
If you are going to re-spring the rear shock you should give some serio0us thought to a re-valve too, I'd imagine works could handle this task, if not TCS certainly can. Good luck.