Rehearse the upgrade on real data before you commit to it
The major version upgrade is the work everyone postpones, because the first time you find out what breaks tends to be the time it matters.
Version is set per instance, not per project. A staging instance can sit on a newer major while production carries on as it is, so you can run the upgrade as many times as you need before it counts.
How the rehearsal goes
- Create a staging instance pinned to the version you are moving to.
- Rebuild it from production. Real volume, real edge cases, the records that were entered wrong in 2019.
- Run the upgrade and watch what breaks. Usually custom modules, occasionally the data, sometimes a view nobody has opened in a year.
- Fix it, rebuild, run it again.
- When a run turns up nothing new to fix, you're ready to move production.
Let the people who use it try it
The staging instance has its own address. Hand it to the people who are in Odoo all day and they will find things no test script was going to, while nothing is committed and the only cost of being wrong is another rebuild.
The rebuilt copy is neutralised, so it won't email your customers or take payments while people click around.
Worth knowing
Production does not move until you move it. A staging instance on a newer version changes nothing for anyone else on the project.
Staging backups are kept for a month. Production backups are kept for as long as the project exists, so the state you are upgrading away from stays available.
The upgrade itself is still Odoo's upgrade, with whatever your own modules bring to it. What changes is how many times you get to run it first, and that you find out what breaks on a normal Tuesday afternoon rather than during the cutover.