The guess about Akiho knowing more than she's letting on is interesting
They're in the same class, she probably knows the both of them
Losing their potency would most likely be in the form of kissing not resulting in swapping at all, or maybe all the time.
It's not like it's an illusion, that losing its efficiency would result in people seeing through them.
Think of it as one's mind being in the other's body. Visually only, you can't recognise whose mind is in whose body.
i completely understand the concept of mind swaps and thought about how and why over kissing would/could affect their swapping ability, but one page in particular caught my attention. something about when Ichonose is in Natsuko's body made me think that her breast size was smaller. maybe it's how she carries herself or maybe it's a fluke and i'm not paying enough attention, but that's why i added the possibility that the swaps were becoming less potent as an option.
Body swapping/mind swapping is the same thing, I just wanted to describe it otherwise like you know, trading the physical, the person being more defined by their psychological existence, their conscience and whatnot.
I think you're overthinking it. The author always drew their boobs that way, check the first chapter. In the bath, Haruko isn't really flat, and with clothes, all the girls with big breasts arent depicted with full blown big ass breasts either.
Cus or else it'd be a terrible mess, if body swapping became only swapping the main traits, certain features being kept. I don't like to think about it, seems too fucked up, like they swap and Haruko's like, blonde or something, and becomes a B cup.