I was messing around in Heart of Fear yesterday (wanted to clear it on Heroic for xmog) when I noticed that upon switching difficulties - the trash respawned.
- Character that has cleared HoF normal
- First Boss room, pop Potion of Luck kill all the mobs downstairs (except Boss ofc)
- set HoF to hc
I then went onto another toon of mine (Deathknight) who also had cleared HoF on normal - I went into the very first boss room and cleared the group in the middle and those on the plateaus - set the difficulity on heroic and there you go, the trash respawned and you're able to kill them once more - I have done this 15 times in a row and it doesn't stop. Seems like this is an endless farm. The only limitation I had was having full bags.