After thinking a bit more about faction switching mechanics, I think we should be very careful about introducing them into the game.
For faction switching from villager to vampire, this might have the effect of accelerating the game into the favor of the vampires. Thus, I think we need to start with relatively few vampires and limit their conversion abilities in some way.
For faction switching from vampire to villager, I think we might want to avoid this altogether. First, it has the potential of revealing all of the information all at once to the villagers. If the doctor makes a successful heal, then the cured player should immediately reveal the identities of the vampires on the next day. Even if the cured player is then immediately executed, that would only confirm that the identities (if player factions are revealed at death). Second, the presence of this mechanism might create a perverse incentive for a vampire to actively betray their team, say by trying to hint to the doctor that they should be cured, and then winning as a villager after being converted. In fact, when cured, I think that new villager would want to reveal the vampires and then actually request to be immediately executed in order to prevent being converted back to a vampire, which are now at a distinct disadvantage.