At present, NSW LRS does not pass Lease Interests to PEXA as part of Registry Information. This gap will be rectified as part of R10.1 (Aug 2018) when NSW LRS update their platform to pass this data to PEXA.
Consequently, even though PEXA currently has the capability to Discharge a Lease, PEXA is not being provided the Lease in order to be able to Discharge it.
As I understand the scenario, you need to Discharge the old lease and register a new lease.
In short, today in PEXA you will be able to register the new lease. But you will have to revert to paper to Discharge the old Lease.