At least according to the folks here:
http://www.reddit.com/r/DestinyTheGame/comments/3l9l6z/you_cant_equip_armor_from_other_classes_but_you/cv4hhis

If you're buying a year 2 version of a year 1 weapon you already have. I'm pretty sure it's just a shard and some glimmer if you already have the year 2 weapon.

I bought the year two celestial nighthawk for 125 legendary marks. You have to have a year one already or find a year 2 exotic.

After you have the year two version unlocked then you can rebuy for an exotic shard + glimmer.

If you're buying a year 2 version of a year 1 weapon you already have. I'm pretty sure it's just a shard and some glimmer if you already have the year 2 weapon.

Anyway, I leveled it up using some motes and took it into Court of Oryx where we failed level 2 miserably. Here's the crap it's got.

The middle ones are rifled barrel in the center and braced frame at the bottom.

The big sell here is that it's an elemental primary, so that's nice. I think it might be the only one of those in a normal exotic, but I'm not sure.

The sights on it are ironsights and look much like No Land Beyond, but a little more svelte. I'm not a big fan of it to be honest.

Even if its stats don't match up that well, it plays very much like an Abyss Defiant. It was doing about 1100 damage per headshot out of the box, and its reload and ready speed are high. It's also pretty easy to hit stuff with it, so you don't need to use Braced Frame at the bottom. When you do, it feels like a slightly slower Hard Light.

The arc lightning perk happens occasionally, about the same as Thunderlord's explosion perk. Maybe 1 in 7 kills, and when it says small amount of super, it means small. It's like the helmet perk. It's also hard to GET those double kills on high level guys because of the small mag size. You need to hit them pretty accurately and well.

My verdict: 2/4 socks. I prefer the vanguard pulse rifle with full auto. It does more damage per shot, has more range, and is more accurate. It's a little slower, obviously, and they have the same magazine. However, for arc shields, obviously you'll want to go Supercell.

EDIT: I'd like to say that the dudes online who got it love the gun. I might just be weird.