Ok, so I got it working by:
- installing openHAB 2.5.11 locally on my Windows 10 machine
- loading my backup of openHAB 2.X
- opening HabPanel → Settings → Edit local panel configuration → Export
- opening the “new” HabPanel (of the openHAB 3 instance) → … → Import
Ta-da, the $OPENHAB_USERDATA/jsondb/uicomponents_habpanel_panelconfig.json file is updated with the correct content!