Changing grades for c.20,000 employees

Summary

Using HCM Spreadsheet Data Loader to update employees' assignment records with new grades

Content

Is the Spreadsheet Data Loader tool the best functionality to update the assignment of c.20,000 employees with a new grade? I have used the 'Update Assignment' tool within 'Initiate Spreadsheet Load which gives me a spreadsheet with the right fields.I am completing the * fields and entering the new grade in the 'Grade Code' field. 

 

I can upload the spreadsheet, but when I use the 'Load Batch Data', the process always fails with the comment of 'No assignment exists within the parameters'.

 

Any idea why this is happening - any guides on using this tool, especially for updating assignments with new grades would be very helpful. Or, an alternative approach of changing grades on mass would be appreciated as well.