We just upgraded to 2015 Update 2 (skipped 1) from 2015.
Prior to the update, we were able to reassign the owner of contacts, opps, etc., in bulk using the Export/Import capabilities
Now, while text fields update fine, the ownerid field (renamed Salesperson) no longer updates using the import tool.
In addition, we get a message that the update run successfully. (e.g. two text fields update fine, the ownerid field does not. The new ownerid records are valid in the system.)
Anyone else experience this? Any solution?