I've been a werewolf, and now I want to be a vampire. What do I need to do, and how do I do that?
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To cure lycanthrapy you must finish the Companions quest line, and finish the radiant quest "Purity". Once you have done that, you can catch Vampirism disease by battling a vampire.
Once you have the disease, you must wait 72 hours, and you will be a vampire.
However once you have cured lycanthrapy you cant catch it again (And so cant be a werewolf)