Yeah, 5-man bosses are much more like raid bosses now as far as abilities they use. There are fires to stand in, and easy ways to get one-shot.
I think they specifically said this is not the case a long while ago. What are you basing this on? They definitely do have different phases as you complete quest lines, but I've not heard of them separating different level characters.
Old World is not being phased in that manner.
Off the top of my head, the only phasing I encountered was in Stonetalon with that one druid school getting blown to hell.