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

Answer: Jokers in viable 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. This includes incorrect exposures made before a player’s hand is declared dead such as exposure made on a concealed hand and all exposures made if a Mah Jongg is declared on a concealed hand. 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);