We did this by accident when we were working on 10 heroic LK, during the role-play right before Tirion got frozen we noticed it was set to normal mode, knowing you could change the difficulty while in combat I called for it to be switched to heroic rather then have everyone jump off to reset it. around the time Tirion was frozen it got set to heroic, the mobs returned to their spawn location when the loading was done and the encounter started with Tirion not frozen. We ended up jumping off during the first phase due to wanting to kill it on heroic, so I'm not sure how the encounter would have played out like that.
I'm not sure if this needs to be timed but I'm pretty sure this will work if you start the RP on heroic then change it to normal. If anyone has a heroic Lk lockout and would confirm this that would be great.