I experimented more and loaded a previous save where she still had vampiric features, then reloaded the save again. Her features were now of a normal vampire again. So, now I can switch Serana from cured features to vampiric features as I please, while still maintaining the option to be turned if I ever want to.
Thought this might be useful for those of you who wanted to cure Serana but not lose the abliity to become a Vampire Lord.