Let's be honest, if they ever wanted to make a family that wasn't just them and a horde of hangers on, Fooley would be best suited for the motherly childbearing role, so this setup works just fine.
Now the question is if it is just a physical and aesthetic change or a full bodily function change. Like if they fuck, does fooly get pregnant? Or is it only good for physical buffs to strength/mobility?
But the really importain question is how the succubus/demon and the other knight girl will react to seeing them as the opposite sex.