Migration from ML2/OVS with iptables_hybrid firewall driver to ML2/OVS works and should be supported. We need to test it officially (but most of our d/s CI is already testing that). We will also need to update our existing docs. Currently in https://access.redhat.com/documentation/en-us/red_hat_openstack_platform/16.1/html/networking_with_open_virtual_network/migrating-ml2ovs-to-ovn we have info that such migration isn't supported (point 3.3. and preface to the article) we have clear statement that such scenario isn't supported. We should change that to say that it is supported and we should explain what are the cons of that (additional linuxbridge between br-int and VM).
Was verified on RHOS-16.2-RHEL-8-20210929.n.0. ovs2ovn migration procedure completed successfully. VMs that were created before migration to ovn remain accessible. Was verified also RHOS-16.1-RHEL-8-20210928.n.1. Tested also that live migration of the VMs that were created before migration to ovn was successful.
Note: while OVN migration procedure itself is passing successfully and workload VMs are usable after the migration,the following issue was found https://bugzilla.redhat.com/show_bug.cgi?id=2011450
*** This bug has been marked as a duplicate of bug 2075039 ***