wow, I've got to try that also, I forgot about that recipe. Unfortunately I can't answer your question other than to say that kefir will ferment a lot of sugars successfully. the question is whether it'll change your kefir culture due to contamination or interactions from whatever yeast makes the sweet potato fly in the first place. Sometimes microorganisms outcompete other microorganisms and cause some changes to a culture like kefir. I'd try it with just part of your kefir grain collectoin, once the grain colony is big enough to split.