There isn't an absolute need to replace the housings. It is generally a good idea to replace them anyway. Modern lined housing will reduce the cable friction somewhat. Also the plastic covering may be starting to degrade by now.

If you reuse the old housing, they should be pulled out and cleaned. Any damaged ends should be clipped off, and all the ends should be ground or filed flat again. Don't forget to put back any ferrules. Grease the new cables generously, and reinstall.