I just signed up the Platinum challenge yesterday, and here's part of the confirmation email I got back from AA. He's super nice, (he was even calculating for me whether I should sign up for the Gold or Platnium challenge.)and he assured me that only challenges start on Jan 1 are subjected to the change. Any challenges that are signed up this year is good even you're taking flights next year(since I'll be fullfilling my challenges only with next year travels, so I was very concerned and was very particular about this and he reassured me)

hope this help.

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Use American Airlines (of course!) and other elite-qualifying carriers to maximize your earnings. These elite carriers are Aer Lingus, British Airways, Cathay Pacific, Finnair, Iberia, LAN, Qantas and Alaska Airlines (and their eligible affiliates). When you earn miles on AA or on these carriers, you also earn points. Only points count toward your Challenge. (But remember that there are some fares on these carriers that are not eligible to earn miles or points. You can easily check out these rules on aa.com/ aadvantage when you select ‘Partners and Mileage Programs’ and then navigate to the particular airline.) And please note: For Challenges that start on January 1, 2007 and later, only points earned on American Airlines, American Eagle and AmericanConnection flights (including AA codeshare flights operated by other carriers) will count toward Challenges.