The Environmental, Planning & Assessment Act requires new work (renovations alterations of a structural nature) to comply with the BCA. The BCA only requires the new work to comply with the BCA therefore Section J only applies to the new work. You do not have to bring the whole existing building into compliance as would be required in the case of the fire provisions.
The only exception to this is where the renovations or alterations are so extensive that the certifier requires you to bring the whole building up to current BCA standard. In which case Section J would apply to the whole building.