Stepping up from 400 to 800 is not as easy for some as others. It might take a year or two of more specific training for a 50/50 aerobic/anaerobic plan to be effective. Some athletes are not mentally or emotionally capable either. Some coaches don't employ enough strength work or speed endurance either thinking they can get by on breaking up the training in smaller chunks especially in the first season of the change but thats just trying to overcome athlete apprehension.
For all of those that thought people like Jeremy Warner or the 400 hurdles types can just waltz into an 800 without a complete overhaul and still achieve their highest potential are just dreaming. Those athletes are maximizing their physiological abilities and I guarantee you their tanks are empty at the finish line of a 400/400H.
Think about how specialized the events are becoming. The days of Snell/Juantorena/Coe etc...are pretty much over with exception to maybe very few individuals. Yes, Rudisha, Brazier and the like might be competitive at distances above or below their specialty but they aren't likely to medal in an honest race.
If the athlete is agreeable to changing events or the strategy is to just avoid stronger competition it isn't likely to workout. I'd say its probably a lot easier transition for 1500/5000 runners like Coghlan or even Houlihan.