I just lowered my Air 2012 Vengeance easily. It was a matter of taking out the air rod, and shifting a spacer down the shaft. It took me half an hour, and I am in no way a suspension guru.
Coil forks however, are a whole different ball game. You have to either, jam spacers under the coil, so it limits the movement and the uppers can't go up more then 100mm. But this will cause the fork to ramp up under compression. To avoid this people cut the coil, which takes a lot of dicking around. Do-able? Yes! Would I do it? Dear god no, i'd balls it up. I would however send it to a fork shop, NSDynamics, Tekkin etc etc, and let them work there magic. Cause you will either end up with a fork that feels terrible, or possibly destroy them in the process.
If you are so inclined to go down the cutting/spacing path, search for a guide to lower a coil fork, like a Fox Van 36 or Totem. The internals and general process will be relatively similar.
