title: Changing a defined column using sqlalchemy into a longer-length unicode
body: <p>I have a table defined in my app like this:</p>
  
  <pre><code>users = Table('users', metadata,
      Column('user_id', Integer, autoincrement=True, primary_key=True),
      Column('user_name', Unicode(16), unique=True, nullable=False),
      Column('email_address', Unicode(255), unique=True, nullable=False),
      Column('display_name', Unicode(255)),
      Column('password', Unicode(80)),
      Column('created', DateTime, default=datetime.now),
      mysql_engine='InnoDB',
      mysql_charset='utf8',
  )
  </code></pre>
  
  <p>However, after developing for a while, I want to change user_name to a longer length, such as Unicode(255). As I understand it, this definition is run at first start-up, and so just changing this line wouldn't work for existing databases. It needs to migrate to this new definition. How do I go about converting already created databases into the new, desired definition?</p>