A compromise:
1.) Just implement a separate Madstone Rage clicky (and Madstone Reaction proc if it still stacks) to use the new nerfed rage and penalties.
2.) Apply this new clicky and proc to the upgraded Madstone Boots (+6 DEX) to use this new rage and corresponding penalties.
3.) Leave the old Madstone Boots (+4 DEX) with the old Madstone Rage clickie and proc.
Conclusion: You implement this new change as an optional upgrade specifically for casters. Melee is left with the old boots to use as they see fit. No, you can't repair the damage done to characters who have already upgraded their Madstone Boots without a lot more work involved in creating a separate item. But you CAN limit this damage by still allowing melee classes to access their old Madstone Rage. Everyone is happy and the popularity of the Madstone Boots is doubled now that caster's have a great use for them!
Genius. This is a work of genius.