Personally, I would rather she change into the uber-hawt Midna that you see at the end of Twilight Princess. Would be kind of weird, but they could just say that they're merely "changing places using the power of the Twilight Realm", rather than them actually sharing the same body, as was the case with Sheik/Zelda last time.
And to me, this kind of thing would make sense, weirdly enough. 99% of the people following this are pretty much expecting Sheik. If that be the case, then really, what would be the reason to hold their tongue on it so badly? Re-design or not, it wouldn't really be worth the trouble of basically going "LOL, you guys were right, it IS Sheik after all for the Down+B! XD" after months of keeping it a secret. There's gotta be something much more profound in the mix.
Now, whether this means saying good-bye to Sheik-y for good, I couldn't tell ya that. Maybe it's a thing of it being a separate character all to itself now (and thus bumping up the total Zelda allotment to at least 4, not counting WW Link). But again...I think Sakurai is being sly with this for a reason...and not necessarily everyone may be pleased for that reason...