To be able to do testing with you I switched to RA router mode "managed" and enabled DHCPv6 service (ISC DHCP for now) on the LAN interfaces.
I do not think that the communication DHCPv6 service to the clients on the LAN subnets is the initial/first/main issue.
The pfSense interfaces do not get an IPv6 address with the new prefix and the new prefix is also not listed in the DHCPv6 service config screen.
To resolve both issues manually as a workaround I'm going to the WAN interface and save and apply.
Afterthat everything is correct:
Interfaces got a valid IPv6
new prefix is listed on DHCPv6 service config screen
clients get a new IPv6 IP with correct prefix assigned
client have Internet connectivity.
This is listed in the system logs while doing the "WAN interface and save and apply."
68c80eb0-60f7-47fe-8250-a57e891e4056-Pasted Graphic 4.png
I assume not all tasks are required on a upstream router IPv6 prefix change.
But some needs to be initiated automatically to allow client IPv6 communication again.
Thanks for your support.