title: SchemaUpdate does not drop tables or delete columns
body: <p>I am using SchemaUpdate to make changes to the database based on some configuration.
  It works fine when new tables or columns are added.
  However, it does not work when columns are deleted or tables are dropped.
  The mapping file does reflect these changes, but the SchemaUpdate does not seem to recognize this.
  I don't want to drop the tables and recreate them, as I want the data to be retained.</p>
  
  <p>Does anyone know if this 'Delete and Drop' functionality is supported by SchemaUpdate?</p>
  
  <p>Thanks</p>