Retina, 40 bytes

T`l`L`_.
(?<!_)([A-Z])
_$L$1
_(?=[A-Z])

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Explanation

T`l`L`_.           # Replace lowercase letters with uppercase letters when after an underscore
(?<!_)([A-Z])      # Replace any uppercase letter not following an underscore
_$L$1              # with an underscore followed by the lowercase version of that letter
_(?=[A-Z])         # Replace an underscore followed by an uppercase letter
                   # with nothing