Minecraft wiki entry for JSON_format, associated text:  conditions:
 villager: The villager that is the result of the conversion. The 'type' tag is redundant since it will always be "villager". May also be a list of predicates that must pass in order for the trigger to activate.
All possible conditions for entities[show]
 zombie: The zombie villager right before the conversion is complete (not when it is initiated). The 'type' tag is redundant since it will always be "zombie_villager". May also be a list of predicates that must pass in order for the trigger to activate.
All possible conditions for entities[show]
 player: The player that would get the advancement. May also be a list of predicates that must pass in order for the trigger to activate.
All possible conditions for entities[show]