What ever you do, they need to look the part...
If they are gonna be loyalists turn renegade... then they need to be identified as such for official events...
Painted 8 cross star... eye or horus for undivided... even SPIKY BITS, or replacing a squad of marines with some chaos models or replacing some shoulder pads or back packs.
If you want to do Soul Drinkers they should be modeled as such... even they have extensive mutations...
Etc. etc. etc.
If you want to do minimum work and just write some fluff about how these really clean looking guys are actually chaos for official GW events... it's not gonna get you any brownie points...
I don't think you a powergamer... you just want to experience another army, cause being stuck in one place gets boring over awhile...
From my experience... I started playing Deathguard and still am... certainly I haven't been alive as long as you... but for 6 years I havn't wavered....
Right now, I'm converting Chaos Tau and Chaos Eldar, both of them opperate from their respective codicies and also can be 'adequate' 'count-as' in my chaos force.
I bridge the gap between the (for example) Tau and the Chaos codex... like having my shas'o corrupted by nurgle and have alot of tenticles and pus coming out of him and lots of Nurgle insignia over him.... he works as my Shas'o in my tau army and an obliterator in my CHaos army, due to the multiple weapons on him.
For my Chaos Eldar... my shining spears are modeled in theme to my plague marines, esp. the head gear.. their jetbikes are corrupted and have growth on them, and lots of eye coming out... they opperate as shining spears in my Eldar army and will represent Chaos Space Marine 'Bikers' in my Chaos army...
Shameless:
http://www.40konline.com/index.php?topic=145682.80Obviously for some I'll have to ask my opponent first... but otherwise, most of my conversion work both ways....
Anyway, good luck with it... Chaos is an awesome army.
My 7 Cents.