The casino has the right to change the dice whenever they feel like it. But usually this happens whenever craps players start winning a lot of money. Superstition is common for casino supervisors too.
The exchange is never noticed by the players and the shooter.
There are other reasons the house changes the dice:
They find the dice too old and the edges too worn down. They change the dice when nobody plays at the table.
The dice can be changed with every new shift or a crew of dealers. The player himself can ask the casino to change the dice. They will hardly refuse, especially if the player is a high roller. Of course the casino is under no obligation to change the dice just because a player asks. One of the dice can get lost after the roll. But in reality it happens very seldom.
Note: All dice sets are numbered and usually have the logo of the casino and the city and state location of the casino imprinted. Dice are cancelled after use with a device that imprints a small circle in the centre on the number 4 edge.

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