I just got a reply from Disney on this. Here is their response:
We are pleased to learn that you are planning to visit the Walt Disney World® Resort and appreciate your desire to make your visit as Magical and enjoyable as possible!
While we still have not been told a opening date for the Rivers of Light yet. Disney's Animal Kingdom will start staying open later Memorial weekend. From that weekend on Disney's Animal Kingdom will stay open late, all the attractions will remain open, however like today Rafiki's Planet Watch, Pangani Forest Exploration Trail and Maharajah Jungle Trek, will continue to close at sunset. Please check out the Disney Parks Blog for all the updated information on all the new nighttime experiences that will start Memorial weekend. Please keep checking the park hours for fall as the website has not been updated that far out at this time.
Please keep checking the Disney Park Blog to for undated information on the Rivers of Light and the new opening date.
We hope this information helps with the planning of our Magical visit!