There have been people who have "moved down" in distance with success but I don't know about that far.
This isn't really "distance running" but Usain Bolt thought he was a 400 man then figured out he was a 100/200 man. Michael Johnson also started at the 400 then moved to the 200, but he then returned to the 400. The 400 and 200 are events that don't have a lot of crossover. Michael Johnson was an exception there.
Ritzenhein pulled his 12:56 5k off of Marathon training. That's a long distance road race to a middle distance track race.
I don't know of anyone moving from 800 to 400 and doing well internationally, I do know of a high school runner who moved down pretty successfully, but the 800 remained his better event. The "brojos" probably know this person, because he went to Cornell.
With the 1500 and the 800, I looked up Sebastian Coe but he started out as an 800 man then moved up. Alan Webb did well moving down, but he remained better at the mile. Nick Symmonds did both events but he appeared to always be better at 800.
The majority of people above the 400 who change "main events" seem to move up rather than down.