Our preferred upgrade path is to upgrade civi1001 from Debian Stretch to Debian Buster. Note this will also upgrade the redis client, mariadb client, python, etc. We can do this in-place with a dist-upgrade and a couple reboots, but there's no rollback option if we encounter issues. So we should do some testing with the codfw server first.
We have a new server nearly ready for install at codfw, we can image that with Debian Buster and use it to test out the new build. That part should be done in a week or two.
After that, the in-place upgrade for the active server at eqiad requires roughtly 1 hour downtime for the OS upgrade, and some additional time afterward to make sure everything is working properly.