In an E3 interview on youtube
E3 - GameSpot Stage Shows - Bethesda Booth Tour - Skyrim Dawnguard

The Skyrim associate says that we can switch from vampire lord back into werewolf as freely and often as you want... how do you regain werewolf status?

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I don't think that you can, I'm pretty sure that Harkon says that getting the vampire lord ability overrides your werewolf ability, and im sure the reverse is true if you do Dawnguard before the companions

Talk to Aela get the disease back. You can only be one or the other though.

And talk to Serana to get turned back into a vampire. I don't think you can do this more than once though.

You can, if you talk to the Companion chick that first made you a werewolf. Im not certain its "has often and freely as we wish" but we can change atleast once. (No idea how you regain vamp lord but i have heard its possible too).