Offline upgrade from Versions in range [5.0 - 5.1.0] to any higher version will need Drop and Recreation of indexes due to MB-28956 or MB-46776. Doc update is needed to mention the same.
As mentioned in comment
MB-28956 Int64 support in Spock breaks GSI upgrade
MB-29157 CLONE [MB-28956] - Int64 support in Spock breaks GSI upgrade