Not relearn; just perfect. As was implied in Welcome to Republic City, she's already mastered the physical aspects of Firebending, Waterbending, and Earthbending, but she's been completely ignoring the spiritual component to it. That's probably the reason why she's never gone into Avatar State despite being in grave danger a couple of times now, and why Aang is trying to contact her. I think once she goes on that spiritual journey, an entirely new facet of Bending will be unlocked for her.
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Cookies and cream! And cookies!