title: Rails migration for change column
body: <p>We have <code>script/generate migration add_fieldname_to_tablename fieldname:datatype</code> syntax for adding new columns to a model.</p>
  
  <p>On the same line, do we have a script/generate for changing the datatype of a column? Or should I write SQL directly into my vanilla migration?</p>
  
  <p>I want to change a column from <code>datetime</code> to <code>date</code>.</p>