In the process of building the Salamander Librarian I'm working on, I wanted to realy reinforce the "librarian" idea. You usually see lots of cables and such on these guys... more than the average Marine.
I decided I was going to do this with his backpack as well. Problem was, only the Chaos backpacks have the cool cables running over the tops of them. Time to make my own version then. This kind of conversion fits well because it's very subtle. It's just a slight change to an easily identified piece of equipment that will make this guy stand apart from other models.
How it was done...
You'll need two backpacks, one Chaos backpack with the cables over the top and one Loyalist version with the regular vents.
Cut the vents off both backpacks.
On the Chaos version, trim down to the red lines so really all you have left is the "body" of the backpack.
On the Loyalist version, carefully trim away until you are left with the entire vent assembly to include the angled mount.
Then it's just a matter of gluing the Loyalist vents onto the Chaos frame. If you're careful and you took your time, you'll only need a tiny bit of greenstuff to fill in any gaps you might have around the seam.
Make sure your vents line up perfectly when you're gluing then on too. You don't want one side pointed in one direction and the other facing another way.
My Project Link: Old School Librarian