Possible Clues:

• Duty to get one hundred and eighty? Case of poacher turning gamekeeper!
• Change in personal relations suggesting trap?
• It's a case of poacher turned gamekeeper for actor synonymous with Trading Places?
• For character in Trading Places, it's a case of poacher turned gamekeeper (4,8)
• 'Mr. Mom' plot premise (and what the answers to the starred clues have)
• Psychodrama technique ... and a literal hint to what's hidden in 19-, 32- and 39-Across
• Plot device used in 'Freaky Friday' ... or a hint to the interior of 20-, 26- or 42-Across
• Assumption of the diametrically opposed function