It's not an easy answer.
If a set is not popular and folks who do play it are asking for changes there is more room.
If a set is popular the change tolerance shrinks dramatically.
Guidance in my opinion:
A tweak is adjusting a value related to a power.
A revamp is removing/replacing powers, impacts to functionality, shuffling the order of powers. (this is where guidance typically is to tread lightly)
It's always easier to give than take away.