Depends on how far gone they are. If worn back to the retainer hole, you are better off replacing them. Otherwise build up with 7018 rod and grind to fit. If you use hardface rod, chances are that the entire shank will become too brittle and break off once you install a new tooth and begin to use it. 7018 is easier to weld with as well over the hardface.
You can use an Oxy/Acetylene torch with a "scarfing tip" to remove the old shanks. It's not as fast as the arc-air nor quite as delicate, but it works.
Best bet is to not allow this to happen. I know. I know. It does and it's unfortunate, but, it costs big bucks to fix and for the loss of production time.
Good Luck!