When you replace flooring...?
Taylor no you do not have to replace the sub flooring unless it is bad only if it is bad just make sure your sub floor is tight not loose otherwise you wind up with squeaks an that will annoy the hell out of you the way to tighten your floor if it is loose is to screw it down with wood screws and make sure they are counter sunk Good luck with your project oh and btw your should be straight or else your floor will look like the ocean with a good waves if you need to replace sub flooring make sure you glue it down with a tube of liquid nails sorry for the book lol
Usually as R T said if it's in good condition you can leave it like that which should be under normal use, but I do suggest you to seal it if it is not and depending on how many years since it was originally install that carpet, maybe reapply seal is it has it.
Only if the subflooring is damaged or uneven or weak or warped. If it's flat and solid, just reuse it.
nope, make sure you deal with any damage, leveling, squeaking, ETC.. while you have it open.