Sometimes just opening the case and flushing with spray solvent frees up the sticky little ratchet parts. I resurrected my current shifters from the junk bin by doing this. As far as replacing, you can get away with using the old housing if it's not trashed, but for a few bucks you might as well take the opportunity to replace with new. Sheldon has a wonderful article on housing installation. BTW some shifters include new housing.