When you change the HSRP version, Cisco NX-OS reinitializes the group because it now has a new virtual MAC address. HSRP version 1 uses the MAC address range 0000.0C07.ACxx while HSRP version 2 uses the MAC address range 0000.0C9F.F0xx. HSRP supports interface tracking which allows to specify another interface on the router for the HSRP process to monitor in order to alter the HSRP priority for a given group.