Question: What are the scenarios for changing an exposure before I discard?

Answer: Below are the two scenarios during which you can change an exposure PRIOR discarding:

Scenario 1: If a player inadvertently exposes the wrong number of tiles or exposes too many jokers, they can add to or take away from the current exposure as long as they have not discarded. In other words, you can change a Pung to a Kong, Kong to a Pung, Kong to a Quint, Quint to a Kong, etc.

Scenario 2: If a player puts the wrong tile atop their rack, let’s say you called a 1 dot for exposure but put up a 1 bam,
you can put the 1 Bam back in your rack and correct it with the 1 dot.

Source: Please note that NMJL has recently clarified this rule during a phone call. And, further explains the rule found in Mah Jongg Made Easy page 15,  para.2 2020 edition