Hey,

Regarding rollaback of upgrade in OSA we indeed don't have any good
established/documented process for that. At the same time it should be
completely possible with some "BUT". It also depends on what exactly
you want to rollback - roles, openstack services or both. As OSA roles
can actually install any openstack service version.

We keep all virtualenvs from the previous version, so during upgrade
we build just new virtualenvs and reconfigure systemd units to point
there. So fastest way likely would be to just edit systemd unit files
and point them to old venv version and reload systemd daemon and
service and restore DB from backup of course.
You can also define  <service>_venv_tag (ie `glance_venv_tag`) to the
old OSA version you was running and execute openstack-ansible
os-<service>-install.yml --tags  systemd-service,uwsgi - that in most
cases will be enough to just edit systemd units for the service and
start old version of it. BUT running without tags will result in
having new packages in old venv which is smth you totally want to
avoid.
To prevent that you can also define <service>_git_install_branch and
requirements_git_install_branch in /etc/openstack_deploy/group_vars
(it's important to use group vars if you want to rollback only one
service) and take value from
https://opendev.org/openstack/openstack-ansible/src/tag/26.0.1/playbooks/defaults/repo_packages/openstack_services.yml
(ofc pick your old version!)

For a full rollback and not in-place workarounds, I think it should be like that
* checkout to previous osa version
* re-execute scripts/bootstrap-ansible.sh
* you should still take current versions of mariadb and rabbitmq and
define them in user_variables (galera_major_version,
galera_minor_version, rabbitmq_package_version,
rabbitmq_erlang_version_spec) - it's close to never ends well
downgrading these.
* Restore DB backup
* Re-run setup-openstack.yml

It's quite a rough summary of how I do see this process, but to be
frank I never had to execute full downgrade - I was limited mostly by
downgrading 1 service tops after the upgrade.

Hope that helps!