I need to change roll of some bones in my rig and would like to save my weight paint so I does not need to re-do it again. Is this possible. I know that I can change roll of finger bones with out deforming mesh but I cant to the same on legs :) Any thoughts :)
Changing the roll of bones will not effect your weight painting and your model in rest-position will be exactly the same.
What will change is your bones pose transformations -> your armature in pose mode will look different. That's because such transformations are in local space and changing the roll of bones changes the bone's coordinate system base around it's Y axis.