The steps to resolve this issue...
Created Maintenance Interface class 3 address dhcp enabled on master (got sync with slave node)
Backup Full Configuration of Slave Node
unplug all interfaces (LAN, Wan, Sync)
Restored Slave config to Master node using maintenance interface
Changed all interface IP addresses of all wan, lan, vlan (Previous master node's Addresses)
Changed all virtual IP's Skew from 100 to 0
Changed all DHCP enabled Failover peer IP addresses
Reboot
Enter persistent carp maintenance mode
Plugged-in lan (Lagg) interface to check
note: It worked and all carp interface status changed from INIT to Backup.
Plugged-in all cabled wan, sync
Master node's H.A enabled and cinfigured
Note: sync wasn't working not with admin account may be because i have changed sync password (for sync account). So changed Sync account password on both master and slave.
rebooted Master node and it worked.
Tested. All is Good now.
I'm still not sure what went wrong while upgrading Master node whereas slave node worked perfectly after upgrade. Clean installation and restoring previous saved configuration has also failed.
Anyways.
Thank you very much pfSense and netgate team for making such a wonderful firewall and keeping it open source.