I've just been looking into this too. I've never touched any of the shocks on my M8, and I'm sure they're in need of attention. I'm planning to do the air sleeves on the front shocks on my own, and planning to have the rears rebuilt and re-valved to 2011 specs (or slightly softer) by gas shock repair. (Who was the fat boy that tuned the early Cat floats???)
I may eventually have the front shocks re-valved too, as I'm getting too old for the beating that my sled likes to give me when the trail gets rough. I may try doing the fronts myself though, just for fun... I recently discovered that the 2011+ front shock piston is shared with the fox shocks that are on my '02 Z Sno Pro, and I have an extra set of shocks that I could rob them out of...
I found "rebuild" kits for the air sleeves for $20/shock on ebay, but if the seals are still good, I should think a good cleaning and a dose of new float fluid would be all they need.
BTW, from what I read, adding the air chambers to both the floats and float 2's seems to help the ride no matter what the valving. DeyCore has the front kits on sale right now for $50. Their cylinders are slightly bigger than the Fett Bros cylinders... Just a heads-up.