Juno works only on Ubuntu 14.04 and EPEL 7 systems (CentOS 7, RHEL 7 and Fedora 20+), so if you're running Icehouse on Ubuntu 14 or CentOS 7, then you'd have a chance to migrate, unless you've to build your parallel juno env. and migrate (the default strategy by OS upgrades :-)).

These documents typically are made available as part of the release notes process, which come out close to the release date. Whether one can go from Havana to Juno directly may not be directly documented. For example, the instructions for moving from Havana to Icehouse is here.