Anytime a key is replaced, the one it replaces gets deactivated. If you lose one key out of four, it's a good idea to have the three remaining keys with you so that the CM can replace the proper key. Depending on the mood of the computer, sometimes they have to replace ALL the keys in the set, which would mean the others would be deactivated as well. (Not a good thing when they're off galavanting in the parks, and all of a sudden the keys/tickets/charge cards don't work)
One additional note... make sure your reservation has all the proper names on it. If the reservation is under Bill Smith, and no additional names, Suzy won't be able to get a key replaced. The front desk will need to confirm Photo ID with a name in the reservation for security purposes.