supported are either the current Sonarqube LTS version = 9.9.1 or the latest = 10.1,
means you must at least update to the current LTS version.
Your upgrade path is 8.6 > 8.9.10 (former LTS version) > 9.9.1 > 10.1 (optional)
see Download Previous SonarQube Versions for 8.9.10 Download | SonarQube
As Enterprise customer you have 1 production instance and two test instances.
You should have received these licenses from your Sonarsource sales contact.
Don’t know about the implementation of your custom rules, but chances are the update will have an impact to your quality profiles.
Our Enterprise setup is like that:
1 production instance on 9.9. We’ve decided to go with the latest instead of LTS (only every ~18 month), as language scanners are bundled and we need support for the latest Angular … etc.
Means updating to latest soon…
Test instance 1 = open for all, latest Sonarqube 10.1
Test instance 2 = latest Sonarqube 10.1, only for internal tests of custom + community plugins and custom rules. Before updating production, we switch to a copy of the Sonarqube production database
for testing the update.
Before the update to either 9.9 or 10.1 you should export your quality profiles to xml and save them in Git.
Backup your Sonarqube DB, then test the update 8.6 > 8.9.10 > 9.9.1 or even > 10.1