The temperatures required to get a good molecular bond from the dye will have a damaging effect on the lycra content of the fabric and the elastic in the waistband. It probably won't "kill" the pants outright but it will weaken any part of them that's stretchy.

Also, if you don't use a dye that has a good affinity for the specific fiber mix of the flairs, you'll wind up with a lot of washout and won't really change the look of the pants.

And just because people don't think of these things and I have a college degree in this, do NOT dye in any containers that get used for food or eating nor use any of your kitchen utensils, do not dye without adequate ventilation and do not touch the dye bath or wet fabric with your bare hands. You don't want to ingest, inhale or otherwise absorb any of the dyestuff or dye-laden steam as the chemicals in fabric dyes are really bad for your liver.

I tried it and it really didn't work. They're just about as faded as they before. I ended up buying a new pair (excuse for new pants!) that I wear only for performance and now wear the faded ones for class and practice.