when they replace your phone do they reuse the motherboard?

my son's iPhone 6 has a broken screen and the sync dock does not work so it can't even power on at this point. I brought it in to the Genius bar and they said that because the frame is dented, they can't put a new screen on it. They offered to replace the whole phone for $299. Dose anyone know if they actually take your phone apart and move the mother board over to the new case? Do they try to transplant as many parts as possible or just junk it or send it out for recycling? I'm afraid that since I can't even turn this phone on there may be other issues. I don't wan't them coming back with additional charges because a camera doesn't work or some other issue. Anyone been down this road?