After migrating an NSS32 pool to OES 2015, its media version is incremented only if all the volumes (including the deleted and inactive volumes) in the pool are upgraded to support hard links.
Only the active volumes are automatically upgraded to support hard links. Therefore, to increment the pool media version, activate the inactive volumes, and purge or salvage the deleted volumes. To know more information on the pool media versions, see Automatic Hard Link Media Upgrade.