When selecting to Preview records prior to performing a global change to add attributes, no records appear and no attributes are added.
Click 'Attribute' from directly underneath the expanded 'Attributes' selection. If selecting the desired Attribute name, then a field, an Attribute will not be added. Only records with the Attribute already existing would show to be changed.
If this attribute is already on a record, make sure that ithe attribute is not marked as Unique. (Example: If you are trying to add Registered as an attribute for 2016-2017 but there is already a row for 2015-2016 and the attribute is Unique then records will not appear in the preview.)
Note: If the record is missing an attribute that has been marked as required, the global change wizard cannot add a new attribute A required attribute can exist on a record if the attribute was marked as Required after the record was last saved.