A queen will be replaced in the event that something unexpected happened to her or, more commonly, the queen's fertility and pheromone will decrease as she continues to age. This act is known as supersedure (would be considered an emergency supersedure if the queen were to die unexpectedly).
In an emergency situation, worker bees will transform a cell with a fertilized egg into a queen cell. If the queen is being replaced because of age, the colony will develop a queen cell for her to lay into.
When inspecting the hive for these queen cells, how do you know if the queen has been released or not?
Here are come photos that may help.