What Chaos Legion would you keep loyal in the Horus Heresy & which Loyalist Legion would you swap with to become corrupt?
Personally I would pick the Emperor's Children to save because I love their ideal of 'perfectionism' and their sick royal colours, plus they're the Emperor's Children (remind me why they still call themselves that?) and I would replace them with the Dark Angels, I always thought of them as quite mysterious, that with the fallen I feel if they would turn to Chaos they would fit a unique role with their fanciness and robes.
PS: I added all the former titles if you wanted to say bring back the War Hounds (Luna Wolves FTW!)