We just completed a successful upgrade from 4.5.3 to 6.0.3 at the end of November. It was on a hosted environment with a customized theme, homepage and some small customizations to support Data Privacy concerns. We also migrated from the Essentials Widget set to the Catalyst widget set. ( I can share our experience with that if you are interested.)
I will be happy an answer any questions you may have about the upgrade.
Here are some links we collected that we found useful to reference across the Jive Community.
Internal Community Managers How are you preparing for your next major Jive version? Do you upgrade... items our users identified as most used as part of their navigation, relevant updates, why you should be excited about the update
The missing 'Your View' is sorely missed by several users - although this initially was raised against v5 there are some useful points for transition to v6.
Outlook: Attaching a document located on Jive this has some good ideas on getting adoption of Jive for Office might be good in our comms.
Sample Communication Plan - Jive 5 Upgrade - really good plan with associated documents