Presumably this also works with other methods of charming on NPC such as praying to the right God or using a scroll of taming.
So I brought the Crown Prince all the way to the Castle. But after turning him in I realized I'd given him my enchanted advertising shirt, so I charmed him with the Lyre of Charm in order to get the NPC commands back and get my shirt.
In doing so the dialogue for the other NPCs in the castle reverted to as it was before I had turned him in.
Fortunately I was able to fix the situation and not break the quest line by turning him into his uncle once again.
So I brought the Crown Prince all the way to the Castle. But after turning him in I realized I'd given him my enchanted advertising shirt, so I charmed him with the Lyre of Charm in order to get the NPC commands back and get my shirt.
In doing so the dialogue for the other NPCs in the castle reverted to as it was before I had turned him in.
Fortunately I was able to fix the situation and not break the quest line by turning him into his uncle once again.