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.
Context: For versions older than R2016x, once you have upgraded the product, perform the following steps to migrate the index structure.
<DATADIR>/config
IndexSchemaList.xml
IndexRuntimes.xml
SimpleRowOrientedStore
ItemOrientedStore
AttributeGroupStore id="1"
id="2"
RAMRow
mmap