Upgrading is always a pain: learning from other people's mistakes is the only way to plan for a flawless transition to new releases.
GerritHub.io has been upgraded successfully to Gerrit 2.15 back in April with zero-downtime but some bumps on the road.
See how the process worked and discuss with Luca Milanesio the challenges and tricks to migrate to Gerrit 2.15 and NoteDb without impacting your users:
Replay slides