That’s true.
tbh I would recommend a fresh install of your Checkmk server to prevent a big headache. 
But first export your data or make a backup to do a bulk import of your hosts.
Also take a look at the official guide. → Updates and Upgrades
2.4. Downgrade – falling back to an old version
The process of switching back to an older version runs just like an update. To be precise, omd update does not care whether the target version is newer or older than the current version – thus you can switch ‘forwards’ or ‘backwards’ as desired.
Be aware however, that even if a downgrade to an older version functions wonderfully, an older Checkmk may not always be able to process data from newer versions. A new Checkmk-version may possibly store data and configurations in an extended format that an older version of the software may not understand.
Configurations which are managed in WATO may possibly be converted into a new format once WATO is in active use and can then store the configurations. As long as such actions have not yet occurred, a switch back to an earlier version is generally unproblematic.
Should you be uncertain whether it is necessary to fall back to an earlier version, we recommend to:
- Perform a data backup BEFORE the update.
- Take the time to test the new version before making alterations via WATO.