title: Mysql error 150 on column rename
body: <p>I've an old db with columns'charset to latin1</p>
  
  <p>Now i'm trying to change the whole db in utf8, i've made a script to change all the tables in utf8 like this:</p>
  
  <pre><code>ALTER TABLE `mytable` CHARACTER SET utf8;
  </code></pre>
  
  <p>and all the columns like this:</p>
  
  <pre><code>ALTER TABLE `mytable` CHANGE `mycolumn` `mycolumn` varchar(200) CHARACTER SET utf8;
  </code></pre>
  
  <p>But on some columns (primary keys and constraints, i guess) i'm getting this error:</p>
  
  <pre><code>Error on rename of './test/#sql-5028_217b96' to './test/mytable' (errno: 150)
  </code></pre>
  
  <p>Anyone knows how to get around this?</p>