title: How change a column type in Firebird 3
body: <p>Since Firebird 3, I can't modify a column type.</p>
  <p>Before I use this kind of update:</p>
  <pre><code>update RDB$RELATION_FIELDS set
  RDB$FIELD_SOURCE = 'MYTEXT'
  where (RDB$FIELD_NAME = 'JXML') and
  (RDB$RELATION_NAME = 'XMLTABLE')
  </code></pre>
  <p>because I get ISC error 335545030 (<em>&quot;UPDATE operation is not allowed for system table RDB$RELATION_FIELDS&quot;</em>).</p>
  <p>Maybe there is another way in Firebird 3?</p>