The legacy supervisory controller can be removed completely if the new head end can perform the following:
Occupied / Unoccupied schedules. The N2 field devices normally do not include a time clock so they completely rely on the head end for scheduling activities.
Trending and logging activities.
Global Variable support. For example, an outside air temperature sensor on one N2 device may be shared with other N2 devices on the same N2 bus or with N2 devices on other Metasys networks.
Custom logic coded into the NCM or other supervisory controllers with GPL, JC Basic, or other languages.
If the new head end meets the above criteria and the legacy supervisory controller is removed the S4 Open appliance will act as bus master.