From the blog:
Caitlin Goettler, Staff Writer
July 6, 2017
Hi Chris – if you’ve booked an American marketed and operated flight prior to July 6, 2017 for travel in 2018, you’ll still be able to earn miles. Just submit a mileage credit request after your flight. As an elite member, if you book on American this year and select a preferred seat for a flight in 2018, it will still hold.