I have an older Trek 850 with Shimano acer X rear derailer and altus front derailer. Dog chewed on the grip shifts so I looking for parts to repair with no luck. Seems like in order to replace the shifter I need to change the derailer which means also changing the casset. Please tell me if im wrong. If I'm right will I have any troubles find a casset that will mount to the hub or will that need to be changed also.