User blog comment:SoKoLoV/New Interview with Todd Howard and New Information & Footage/@comment-80.254.148.59-20111021133405/@comment-89.244.117.55-20111022023501

I guess you can revive undead infinite times. They were undead before, so why should they burn to ashes after turning undead again? But reviving a normal NPC whose body isn't used to being undead causes turning into ash.