it's not a habitable living space (loft conversion) so it doesn't need to be plasterboarded
if at some future point you decide to convert it, it would have to be done properly, so the old plasterboard would be removed.
Currently you have a damp problem and need to strip out the roof slope lining
if at some future point you decide to convert it, it would have to be done properly, so the old plasterboard would be removed.
Currently you have a damp problem and need to strip out the roof slope lining