My friend has soft AC ductwork running up an interior wall. He wants to build a wall around it to hide the ductwork. My idea is to pack out the wall by nailing in framing between the ductwork right to the existing framing. My question is, do I need to remove the drywall to pack out the framing, or can I just nail 2x4s to the existing framing through the drywall? I would then drywall over the new 2x4s to hide the ductwork and go from there. I am certain it would be solid, I just want to be sure.
It doesn't really matter either way. You'll have to do the same work to finish the drywall in either case.
If it's an exterior (insulated) wall, then cutting the drywall without damaging the vapour barrier is a bit tricky. I usually try to avoid cutting exterior walls for this reason alone.
Benefits to cutting the drywall:
So unless you have a compelling reason (mostly one of the above), I wouldn't bother cutting existing drywall.