Some parts on a stuntbike are upgraded because they "feel better" or "look better." Upgrading your 2003-2004 Kawasaki ZX-6R forks is common becuase running the 03-04's is a bit like playing Russian Roulette.
We've all seen this classic image of the helmetless, gloveless rider, "JNutz" who became a poster child for the issues you might face running the 03-04 forks. This guy isn't alone though, pleanty of people have suffered a similar fate; their forks snapping mid-endo resulting in a very messy high-speed crash, check the photos.
Mostly recently Joe Tobolic snapped his forks in Florida while practicing for this weekend StuntJam competition - we wish him a speedy recovery. If you're planning on doing endos on your 636 consider swapping to an 05-06 front end or 07-08 ZX-6R front end.
"PSA 03 04 636 forks are bad and they break with no warning." -OMR
Even with the more reliable 05-06 or 07-08 forks you run the risk of snapping them, so regardless of what you run, check your forks every so often and keep them clean so you can spot problems when the begin. If your forks are leaking oil along the base (where they normally snap -check the images) change them ASAP!