Look at the wording here.In post 2, mastina wrote:At any time the numbers between sides are equal (e.g. 9:9, 8:8, 4:4), once per full phase (day/night), the mafia may switch two currently-living players: one from light, and one from dark. (In other words, were numbers equal on all of D1/N1/D2, the mafia may switch during all three times.)
It's not required.