In a multi-rhel configuration, where RHOSP 17.1 computes nodes running RHEL 8.4 and 9.2 coexist, the migration of virtual machines is limited to the following:

1. Live migration between RHEL 8.4 computes is allowed
2. Live migration to a RHEL 9.2 compute is not allowed when the source is a RHEL 8.4 compute node
3. Cold migration is allowed to and from RHEL 8.4 and 9.2 computes

I think this should these limitations should be mentioned in the FFU guide, chapter 11, section 3 [0].

[0] https://docs.redhat.com/en/documentation/red_hat_openstack_platform/17.1/html/framework_for_upgrades_16.2_to_17.1/assembly_upgrading-the-compute-node-operating-system_upgrading-the-compute-node-operating-system#assembly_upgrading-Compute-nodes-to-a-multi-rhel-environment_upgrading-Compute-nodes-to-a-multi-rhel-environment