I changed the accumulators on my S500 last weekend. There are several good posts on this board on how to do it, search for "accumulator" and you should find them.
The only piece of advice that I wish I had before doing it is this: attach the samll hydraulic hose, then the large one, then the mounting screws. I could not get the small hose lined up after I had attached the large one.
As to the fluid, drain the pressure off the system at the valve using the bleed point, attach a hose leading into a pan to collect the fluid. I only had an ounce or two come out of the lines after the pressure was released.