Welcome to Librarium Online!
I'm rather new to 40k and I've decided to go with a DA army, more specifically a doublewing army(first part I'm doing is ravenwing). i was reading the new DA codex and it say that ravenwing attack squad got the option of a bolt pistol or a chain sword. i just want to clarify that i can give all of my raven wing bikers an chain sword, right? other question is can i sub the regular sword arms that come in the ravenwing box for the chain sword seeing as they're both cc weapons?
You can choose to model the bikes with chainswords if you want, but it isn't necessary since it comes with the default equipment. I think WYSIWYG only applies when adding upgrades, not necessarily the model's given wargear. Anyways, the pistol/ccw isn't going to make any difference unless your sarge has some special combat weapon (power weap, fist), and your bikes should always use the bolter over the bolt pistol when shooting.
Second ?: Yes you can. Using say a lightning claw or power fist to represent a ccw is pushing it, but if its just a sword, etc, no one should care. In friendly games, people will usually let proxy/counts-as go as most of your army stays true to appearance.
So, go ahead and give them the swords if you like how it looks ;Y