why would something that is local to 1.2.3 even talk to pfsense to talk to something else on 1.2.3..
"Now we have lots of App and Links with us from long ago configured with IP 1.2.3.5"
You configured stuff to use an IP and not a FQDN?? Yeah fail for sure..
You do understand that a machine especially a webserver for example can have more than 1 IP on the same network right.. So your new server can have 1.2.3.4 and 1.2.3.5 and serve up stuff on port 80 for either of them.. Your server could serve up http (80) on hundreds of different ips if you wanted too, etc..
What you should be doing is redoing all your stuff that have a hard coded IP in them.. This is just bad bad bad idea all the way around!!