She was cured of vampirism? How the heck>?!
She can't *decide* to be human, roflcopter. Sorry, no offense meant. However, you CAN turn her into a human but I have no clue whether or not she can be a vampire again if she has been cured.
Only way to cure serena is after the quest "kindred jugement", after killing Harkon. Upon success, fast travel to any location, then back to serena. Upon doing so, the option "Have you ever thought of a cure?" Will Appear. Upon saying this she will say, "I've never thought of that." Then proceed to say, "You could be your own person again." She will then say, "ok, I will go see Fallon for a cure.".