Question:.  Can Jokers be exchanged from a hand that’s been declared dead?

Answer: Jokers in valid exposures before a hand is declared dead are still available for a Joker exchange.  Jokers in incorrect exposures at the time a hand is declared dead, whether noticed right away or later in the game, are returned to the  player’s rack (slant part). This includes any erroneous exposure that were made prior to the hand being declared dead. For example, exposures made on a concealed hand and all exposures made if a Mah Jongg is declared on a concealed hand are returned to the player’s rack.

Returning the incorrect exposures to one’s rack prevents confusion because there might be valid exposures with Jokers atop the player’s rack.  In the case of a concealed hand that has been exposed in error and declared dead, the entire hand is returned to the player’s rack.    

Source:  Mah Jongg Made Easy (2020 page 16 #3 [b], page 24 #19-21)