Hi Sam,
After you complete migration of all interfaces to a new PI system and will have no necessity in running any of them on an old PI system, you can generalize IS_URL by using it without sub-parameters and pointing to a new PI system. Having done so, you may delete interface-specific IS_URL entries since they will become superfluous (all outbound proxy calls will anyway be routed to a new PI system using default Integration Server referred in IS_URL without sub-parameters).
Regards,
Vadim