Hello Richard,
Classification should not be a problem if you have uploaded a wrongly. Thanks Adobe on this!
1. If you have correct data mapping to the wrongly pointed report suite, please upload the same. Don't forget to check the below box while uploading. It will overwrite the earlier wrongly uploaded data with the current one.

2. If you don't have any file to rewrite the classification, you can use the method you have shared above. Other than key, make everything empty to rewrite the classification using the above method(Don't forget to check the 'Overwrite data on conflicts'). It will take a day, so wait for it.
But you cannot delete values within the key column.
Thank You!
Arun