In a workflow process I would like to change a value to the previous value.  Is this possible?

Here is the scenario.  I have a status field that the users can change to "Completed".  When this field is changed to "Completed" a workflow process executes.  The first thing the workflow does it verify some of the other fields are filled out correctly.  If they are not then I want the workflow to set the status field back to the value it was before it was marked "Completed".