Grats on beathing him. - Now we need to kill the other three...
Can we salvage the wreck and gain some tech? - Yes. for sure.
Does it cost anything to change FC, and can we set them back to PD? - It's just a setting on the Turret's Fire Control computer. I could tell an anti-ship Turret to do a PD's job (although it wouldn't do it very well). Or (as in THIS case) I could tell a PD turret to act as an anti-ship Turret (and again, it will only do a half-assed job).
When I first build a STO, I choose a FC type.
THIS APPLIES ONLY TO STO.
If I choose "Anti-Ship", then I get a FC that fires at
4x base range, against 2x base speed targets.
If I choose "PD" instead, then I get a FC that fires at
2x base range, against 4x base speed targets.
Example:
Suppose the techs that I've learned give me "Fire Control Range 45k" and "Fire Control Speed 5000". (Those are, in fact, my current techs.)
Asking for a Anti-Ship FC on my STO gives me a range of 180,000 km (4x range) and can engage without penalty targets moving at up to 10,000 kps (2x speed).
Asking for a PD FC instead on my STO gives me a range of 90,000 km (2x range) and can engage without penalty targets moving at up to 20,000 kps (4x speed).
Whichever FC I ask for at build-time, I am stuck with it. If I ask for a PD FC for unit X3, then unit X3 will have a PD FC for the rest of the game.
But I can actually USE the unit for whatever I like. If there are no missiles coming toward me, but a nasty enemy ship, then I can tell my PD turret to shoot at ships instead of missiles... or vice versa, tell an anti-ship FC to start shooting down missiles... but it won't be very GOOD at it,, because the unit is equipped with a fire control that was designed for a different role.