Thanks @maverickws . The purpose of this revamp:
they wanted to use only single pair of pfSense so that it can handle the traffics for the whole subnet 172.16.0.0/21. For eg 172.16.1.0/24 for client A, 172.16.2.0/24 for client B, 172.16.3.0/24 for client C and so on.
Initially there are 2 WAN as in 2 ISP, 1 for each pair of the old pfSense unit. So now will be reduce to only 1 WAN (1 ISP).
I will need to create few VIP at the new pfsense as a gateway for each subnet, for eg 172.16.1.1/24 for client A, 172.16.2.1/24 for client B, and assign VLAN to each of the subnet and configure some rules so that they wont be able to communicate with each other.