it depends on how much room there is for insulation. if there is
enough space to add a r38 bat on top of the old stuff, then no need to
remove. but if you are trying to maximise r value in a limited space,
then i would remove the rock wool. also be sure that you have a good
vapor barrier and that any penetrations are sealed well, like ceiling
electrical boxes, bath fans, plumbing vents, and the like.

if you are sheetrocking anyway, it will definitely pay for itself.