Got the same challenge info as everyone else. While they wouldn't give me the Platinum status starting now (the beginning of my challenge period), they did give me gold which is better than nothing (UA silver equiv).
Also, when I hit the requirements of the Gold challenge (7,000 miles) even if I don't make it to Platinum in the challenge window, gold is locked in for the year. Still crappy but a little better to have SOMETHING for the challenge period.
Proof of existing status was not required and, apparently, you don't have to have status at all to do the challenge.
on February 16, 2016