On upgrading a single node cluster from 4.5.1 -> 5.1.0, the ForestDB indexes are upgraded to Plasma. Post performing a rebalance (with no nodes added/removed), the Index Storage Mode list on the UI Settings page still shows "Legacy Global Secondary" option. The indexes are upgraded to plasma and this has been verified from /getIndexStatus endpoint.
If upgrading from 4.5.1 to 5.0.1 and following same steps as above, the option "Legacy Global Secondary" goes away post rebalance. Hence this is a regression.