Chasing existing threads with a sharp tap shouldn't be a problem; all it will do is push any debris in the threads out of the way. If the threads have been damaged by e.g. cross-threading, dented shell, etc., shell material will be removed by the taps, and you can only remove so much material before it becomes a problem. English or metric thread shells with threads damaged to the point where cups are no longer secure can be reamed and tapped with fresh Italian threads, or a threadless cartridge can be used. Severely damaged Italian threads can also be addressed with a threadless cartridge.