V. Basic and easy - Storm Troopers to Chaos Storm Troopers???

[_Toast_]

How about this? Click me!

Failing that, id go for FW, failing that, well im sure if you need something else i can put on my thinking cap...

[Andusciassus]

I'm not sure if you are still working on this or if you've finished, but as this poped up I thought I might give my 2ps.
It's always easy to just give a model horns and say "he is chaos, see" as if that's all there is to it. But I do prefer a more subtle way I guess.
What really seperates chaos from their loyalist counterparts is how viscious and evil chaos worshippers are. They do things no sane Imperial citizen would, and no Blood Drinkers and Fleshtearers are not really sane...
SO, without going to far into the advanced conversions you could just (after defiling the aquilla of course) add some brutal details, like the meat-pieces and knifes from the Kroot sprue. That'd tell any observer those guys are mean, eating raw meat and cutting it from the dead bodies with crude knifes is quite a give-away.
You should also focus some extra energy and attention to the vet sarge as he is the only one showing his face (if you are using Kasrkin that is) so why not make it either very pale or give him horns like Darth Maul? Like a crown of small horns. It'd take some carefulness and patience but it's really not hard to do.
You could also swap one of his hands for a mutated version, or even something from the Zombiesprue.

[slasherama]

you are in luck I tried this conversion not 2 months ago. If you are willing to buy different models rather than the metal storm troopers read on....Ok well first you buy 2 box sets- fantasy chaos warrirors and space marine scout boxed set (with sniper rifles)
now what you do is use the heads from the chaos warriors on the scouts (it really looks good). Defile any insignia the scouts have and cut some tatters into their capes. Alternatively you can just cut the shoulders of the chaos warriror capes and use them for more chaosy goodness.
Now what you want to do is make the hellguns, to do this simply cut the silencer of the sniper rifle and shorten the barrel. This gun now fits perfectly with its description (targeter, attached to power pack...etc) and looks like a nice boarding action short pattern assault weapon
You can swap the scout hands (by cutting off at the wrist) and swap them with the cooler gloved hands of the warriors which also carry brutal close combat weaponry, which well all know is a favourite of our spikey friends.
Adding kroot accesories wont hurt your image...even convert a few rifles
A nice touch is to use a small dot of pva glue in places over the minature and dip them in sand, this dries like a big crusty sore or choas warp leaking out of armour cracks. Which is perfect for nurgle or can be coloured in other vibrant ways (pink and violet are mine) to make the model stand out and represent the other chaos powers.
hope this helps. None of this was done using really any modeling prowes and can be completed quickly and with stunning effect.