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ID84-Commitment to a Joker Exchange
Question: Can I change my mind once I’ve done a Joker Exchange?
Answer: Once a player receives a joker after it is replaced with the same symbol tile, the exchange cannot be reversed. They have committed to the exchange. This ruling is not found in Mah Jongg Made Easy.
Sources: National Mah Jongg League Bulletins Q&A (2001, 2014, 2015)