To answer your question in specific - it depends.
To answer your question in general, Mitel assures their paid clients that "in place" upgrades are supported and will work with no problems. For others who use the unsupported version and install contribs from a variety of sources, the answers are not so clear cut.
A relatively stock install should be little to no problem and the amount of re-installation and troubleshooting after an upgrade increases exponentially with the amount of customization.
While YMMV, my recommendation would be a fresh install of whatever version you chose, apply updates, then reinstall the custom apps you are utilizing.
mySQL databases move transparently, but IMP/Horde, firewall rules, etc are not so trivial depending on versions.
HTH,
-jeff