Bohrium is a Ganeti VM so we'd need a new one to properly upgrade to Debian Stretch. A high level plan would be:
- Create a new Ganeti VM with Debian Stretch
- Deploy Piwik/matomo to it and test that everything runs as expected.
- Plan a controlled outage in which piwik is stopped, and the database is dumped and recreated in the new host.
- Point piwik.wikimedia.org's Varnish director to the new host.