Ok. Thanks for the fast response Are there any concerns if we do a dist-upgrade (do-release-upgrade)? (should we do that before or after the update of ISPConfig?)
Do the OS upgrade first, then ISPConfig upgrade, and of course reconfigure services when asked. Most things update pretty cleanly, though you may have a few things to fix from version updates, eg. apache 2.2 -> 2.4 config. As always, backup everything first.