So disconnected the 4 ports from the HW firewall, connected the ports on the pfsense and reset the IP address in the LAN interface to match that of the H/W firewall.
The IP address changed from … to ... ? (It was my experience that some major configuration changes seem to require a restart to completely clear out the old details.)
At this point the internet goes down and doesn't come back up,
As seen where? from pfSense console? client computer connected to pfSense?
I cannot ping the LAN interface any more and generally bad things happen.
What ping command did you use and what does ping report?
Why do things change so much with just an IP address switch?,
If you want a detailed explanation you will need to provide more details of your configuration and how it changed.
So disconnected the 4 ports from the HW firewall, connected the ports on the pfsense
This suggests you have at least three "LAN" ports on the pfSense box. Unless these ports are bridged you will need to add firewall rules on the ports that aren't THE pfSense LAN port to allow traffic. (The default pfSense configuration is to allow connections arriving on the LAN interface and block connections arriving on the other interfaces.)