Crak the Card
When a hand dies, what becomes of the jokers
A point of contention finds a clear public answer.
A question that stirred debate earlier this year, what happens to jokers in an exposure once an incorrect mahjong is called, has a clean answer in public sources: jokers exposed when mahjong is wrongly called are dead once the hand is declared dead, per Sloperama's long-running Q&A bulletin board.
The short version Prim would give you: jokers can be exchanged from exposures made before a hand is ruled dead, but not after. The timing is the whole rule.
We teach the procedure here rather than reproduce any league's text, and OOTT is not affiliated with the National Mah Jongg League. For the canonical language, see your own materials.