Hello,
If going from 8 speed shifter to a 9 speed,
do I need only the rear shifter changed and keep the original shifter for the front?

If I only need the rear one changed, is it possible to purchase somewhere only the rear one, and not have to buy the whole set?

Can 8 sp. be somehow converted to a 9 sp. (if that is possible, is that advisable?)

Thank you.