I found instructions indicating it might be possible to do a sort of upgrade: Linksys WRT3200ACM & 21.02.0-RC1 popup message - #7 by hnyman
So I renamed /etc/config/network and /etc/config/system to network.old and system.old and flashed retaining the configuration.
This worked as expected. I had to re-create my guest network and attach my guest wireless to it. Then I setup the VLANs for the wired network.
Is it well known with the DSA devices the WAN MAC address changes when upgrading from previous versions? This is problematic with ISPs like Ziply Fiber where after two leases you can't get another until the lease time for a previous lease expires (four hours in this case).
Good news, with this version my WAN speed is still good: https://www.speedtest.net/my-result/d/e50d8588-fa34-475f-b26f-7e6392d3e64d