I just wanted to know what the known issue/s reported when upgrading ZDs from versions before a recommended version to a higher version. I've been seeing this on release notes but the issues are not specified (failed upgrade or successful upgrade but had major issues. Did all that went straight to a higher version experienced issues?
The ZD maintains an internal database which is version-specific. One of the things that occurs during an upgrade is the database schema is updated to match that new version. When upgrading from an earlier release which is not in the supported path, the database transforms aren't understood and so the results could be both subtle and un-predictable (never a good combination..)