May be a dumb question, but lets say my buddy takes a Lascannon as Signature wargear for his Dev. What happens to his HBolter? Can he freely switch to it prior to a mission, as it is all part of his kit, or does he need to pay the Req. Cost to change to the new item?

Not sure on an official ruling, but I would rule that the Dev has both weapons as part of his 'kit'. Obviously, he's gonna have a hard time carrying both the heavy bolter and the lascannon in the field, but he can freely choose which one with which to deploy.

He could bring both if he had a way to transport both. If you took a Rhino for the mission, for example. Or perhaps bring one with a backpack supply and one without, though logical carrying is going to be hard in that case.

what about having a follower, from First Founding, carry the extra weapon for him when he's not using it?

That should work, yeah. A follower carrying the spare weapon. Plus, if the follower is another Astartes (possible with the Greater Follower), they might be able to use the heavy weapon.

I really don't find Astartes appropriate for Followers, with the exception of a Black Templar who is training a Neophyte.