The locking system and alarm will need to be reprogrammed. This will desync the old keyfobs and prevent them from communicating with the car and unlocking the doors. If the car has physical key, the locks and ignition may also need to be replaced. If the car is within its manufacturer warranty period then this work will need to be completed by the dealer, otherwise it may invalidate the warranty. Older cars can have this work carried out by an independent auto locksmith.