I asked a similar question to an IR Examiner a year or so ago, and his reply was there is no set differences training syllabus.
He even said that when going e.g. PA28 -> TB20 (retractable with a VP prop) a ground briefing could be legally sufficient, in cases where the pilot has received the experience elsewhere (e.g. flying with a type-experienced pilot who happens to not be an instructor)
Let's face it, moving up the lever with the wheel-shaped knob on it isn't complicated, and a VP prop isn't much harder. It's the procedures (flying more by numbers) which really change.