Here's an interview with Irish runner Sinead Denny, who started off as an 800m runner but has recently moved down to 400m and has had considerable success and will be challenging strongly for a place in the European Championships later this year.
http://www.therunningreview.com/2014/01/31/athlete-qa-sinead-denny/
Usually there is a lot of talk about athletes who have moved up in distance or who should move up in distance, but you don't often hear discussions about the opposite. What athletes at the top level have successfully moved down in distance, and what current athletes should move down in distance in your opinion?