Please consider also your used database. Depending on the used engine, you will need to use different database drivers on your upgrade road or even need to upgrade the database installation. SonarQube 6.3 is quiet outdated. Maybe 2017, when I started using SonarQube?
I want to add to that and point you to our documentation on an upgrade roadmap, highlighting the database backup. Back up your database before each upgrade, as it can help you understand (and recover from) possible upgrade issues.