I am following the steps for upgrade and it asked me to run nb_clean_datamart_sp I am assuming this is to delete everything from the datamart so that any changes to the datamart because of the upgrade may change some fields in the datamart
You should not have to manually execute this stored procedure.
As you stated, it is a procedure to clear out the Datamart Tables.
The upgrade should handle it automatically if it is needed.
Thanks for the quick response!