I just got the sex tape and was very curious how it works, so I ran some experiments. Here are some preliminary observations:
First off, convert does not mean that it removes anything, so leveling these cannot hurt you.
Second, my strategy has been to only level Bunny and Estelle, so I didn't see any effect from this charm. What it does is when you level up Mizuki (or another girl that adds to click) it also adds to idle.
I haven't worked out the exact formula, but I just wanted to get this out there.