by snowblizz at 15 Dec 2009, 20:11
Currently the Deamon models are exactly the same, as they ahve always been. The boxes just come with round and square bases. In fact last time I looked they had their own "logo", i.e. WHFB boxes have a Warhammer logo, 40k boxes have a Warhammer 40.000 logo. Deamons just said "Chaos".
As to what you can and can't cross over, that depends a bit. There's not a person on this earth that can say if you are using Orc or Ork models (without a very close study), they are purposefully made so actually. Usually they way it goes is that you take models from WHFB and add 40k weapons to them. That's the easy and most common way to cross over.
However I've seen some rather impressive WHFB armies made from 40k models. Often for Deamons though, as they lend themselves to "odder" appearances. And lack plastics kits.
To some extent 40k is more lenient, from a game-mechanics POV, so often you'd see Deamons with square bases in 40k. Round bases being somewhat impractical in WHFB.
Currently the Deamons of Chaos (WHFB) and Chaos Deamons (40k) armybook/codex is functionally identical. 40k have an additional Demaon enginge, the Soul Grinder but that's the only difference.
DoC are a freakishly good army, CD are much less so. The 40k version "suffers" somewhat form a random deployment rule which means you cna't really guarantee to get exactly the units you need onto the table.