I just wanted to confirm what Ghostt said. The PreGen's spring is heavier than the one that comes on the 500, so I swapped it. I used a homemade spring compressor consisting of a couple ratchet straps (Redneck Engineering - Do Not Try This At Home!). Once you compress the spring, you just loosen the locknut and clevis from the shaft, then the spring will fall right off.
The 500's preload adjuster lets you release all tension from the spring, so you don't need a spring compressor.
Here is the process for swapping the spring on the 500's shock, which should be about the same other than needing a spring compressor rather than just loosening the preload adjuster.
Also note that the PreGen's top spring perch is held on the shock body with a circlip. I haven't tried it with the spring still installed, but you may be able to simply pop the circlip off once the spring is compressed. That would allow the perch to move up and relieve the spring pressure, which would then give you easy access to the nut at the bottom.
The NewGen/300 shock has a notched lower plate that just slides in and out once you relieve the spring pressure.