I like the idea. Just to double check, are you thinking of two separate character sheets that you will switch between? I think that could work but I'll have to give some consideration to game balance. As both of those classes have d8 HD they will probably share HP and conditions.
You could roll each night to determine if the change happens.
It is definitely a curse from a roleplay perspective, but mechanically there is not much hindrance there. We will want to add in a drawback of some kind each time the change happens.