We have recently upgraded from Sonarqube 6.7.5 to 8.9.2.
We see that the time effort for some rules have changed and the consequence is that the technical debt has considerably increased.
For example:
In Sonarqube 6.7.5 :
It is normal, particularly for such a big jump, to see changes in rule metadata from version to version of SonarQube. We are constantly re-evaluating our rules and occasionally make changes to the description, type, severity, remediation time, and so on.