When upgrading to cPanel version 100 from a version before 76, cPanel will not upgrade and gives output similar to:
=> Log opened from /usr/local/cpanel/scripts/updatenow (4139) at Fri Nov 12 23:51:09 2021
[2021-11-12 23:51:09 +0800] Detected version '188.8.131.52' from version file.
[2021-11-12 23:51:09 +0800] Running version '184.108.40.206' of updatenow.
[2021-11-12 23:51:09 +0800] Target version set to '220.127.116.11'
[2021-11-12 23:51:09 +0800] E Blocker found: Downgrades to a previous major release are not permitted.
[2021-11-12 23:51:09 +0800] W An attempt to up/downgrade to 18.104.22.168 was blocked. Please review blockers.
[2021-11-12 23:51:09 +0800] E The install encountered a fatal error: An attempt to up/downgrade to 22.214.171.124 was blocked. Please review blockers.
=> Log closed Fri Nov 12 23:51:09 2021
Older versions of cPanel do not recognize three-digit version numbers and attempt to downgrade to version 10 instead of upgrading to version 100.
We've opened an internal case for our development team to investigate this further. For reference, the case number is CPANEL-39256. Follow this article to receive an email notification when a solution is published in the product.
Set the release tier to 'LTS,' then proceed with the upgrade. Once cPanel has been upgraded to version 76 or higher, the release tier may be set back to 'Release.' At the time of this writing, 'Stable' is version 98.