A new index structure called Item Store is available since Exalead CloudView R2016x. It provides better performance at indexing time as the index is now fully generational.
For versions older than R2016x, once you have upgraded the product, perform the following steps to migrate the index structure.
1. In <DATADIR>/config, edit the IndexSchemaList.xml and IndexRuntimes.xml files.
2. Search and replace:
◦ SimpleRowOrientedStore with ItemOrientedStore for AttributeGroupStore id="1" (not id="2")