Hello,
first sorry for my bad english .
you need 2 players , inv hem in group or Raid . put Dungeon Difficulty on Heroic , leave the leader out of instance and with scened player go into instance , before die the Boss , Leader Change the Dungeon Difficulty to Normal , after that kill boss , you will see boss it's drop Heroic item's .
when a leader change Dungeon Difficulty to Heroic Leader receive warning message : Cannot Reset Voilet Hold. There are players still inside the instance , but instance it's change to normal to all player in raid or Group .
when die the boss go out off instance , again change Leader Dungeon Difficulty to Heroic , player go into instance , you will see it's not save , and can repeat more and more and more with out saving instance .
i check it in all Dungeon Difficulty and raid instance and it's working .
thanks