For every data partition undergoing incremental reorganization, a row is stored in sysattributes. Use the dbcc command zapdefraginfo to delete this information from sysattributes before performing a downgrade.
Also in a running server, if the rows with defragmentation information for a specific object are accidentally lost from sysattributes due to unknown reasons, use dbcc zapdefraginfo to reset the extent version information for the specific object so that a later reorg defrag will not fail to consider all the extents of the object.
The System Changes section in this guide provides syntax information.