Fallout – equip the survivors in Fallout 4

It's not possible to do this without any component cost. Scrapping a weapon destroys its mods.

But it shouldn't take long to find a few spare weapons, break them down, and make enough components for a swap at a relatively low component cost. If you can manage to afford the Level 1 Scrapper perk this gets even easier.

It sounds like you might know how to do this already, but in case it helps you or others—let's say you have a Powerful Receiver mod on pistol A, and you want to move it to pistol B.

1. At the weapons workbench, select Pistol A, select the Standard Receiver mod, then choose Build. Though this isn't free, it's really pretty component-cheap.
2. Look in your inventory to confirm. You should see a Powerful Receiver mod in the Mods inventory tab. This mod is now available to apply to other pistols.
3. At the crafting bench, choose Pistol B and navigate to the Powerful Receiver mod. Since you already have one, instead of Build you'll see an option to Attach.

I don't believe there's a way to sell your frames. I certainly haven't encountered any, and the fact that the frame doesn't show up in your inventory makes me pretty confident that I won't find any.

You can get out of power armor underwater, so you can just walk the frames into a river if you want to get rid of them. Alternatively just leave them somewhere. Fast traveling does not drain fusion cores. They'll eventually disappear if you leave them outside of a settlement.

If someone puts them on and you kill them, the frame is not lootable from the corpse, so I guess that would get rid of them.

If you decide not to get rid of them, they aren't completely useless. You can save one for any human or synth companion you take with you.

Settlers will get into a vacant frame if they're being attacked and it has a core, so you could leave equipped frames at your settlements to help your settlers with defense. It's plausible you could order specific settlers to enter them from the workshop menu, but comments indicate that this has not worked out in tests.

You could also outfit different ones for different scenarios, but that's probably not a great use of your time, as it's easy enough to just swap the parts on the frame you normally use.