The formatting of a nice message for this event hasn't been implemented yet, so 
I'll just put an XML dump of the event here.

<ns:fieldTypeUpdated xmlns:ns="http://outerx.org/daisy/1.0";>
  <ns:oldFieldType>
    <ns:fieldType id="4" lastModified="2006-03-01T13:11:43.000Z" 
lastModifier="109" name="Tags" valueType="string" deprecated="false" 
aclAllowed="false" multiValue="true" updateCount="1" size="32" 
allowFreeEntry="false">
      <ns:labels>
        <ns:label locale="en">Tags</ns:label>
        <ns:label locale="">Tags</ns:label>
      </ns:labels>
      <ns:descriptions>
        <ns:description locale="en">A set of tags used to categorise a 
document</ns:description>
        <ns:description locale="">A set of tags used to categorise a 
document</ns:description>
      </ns:descriptions>
    </ns:fieldType>
  </ns:oldFieldType>
  <ns:newFieldType>
    <ns:fieldType id="4" lastModified="2006-03-04T17:50:27.455Z" 
lastModifier="123" name="Tags" valueType="string" deprecated="false" 
aclAllowed="false" multiValue="true" updateCount="2" size="32" 
allowFreeEntry="true">
      <ns:labels>
        <ns:label locale="en">Tags</ns:label>
        <ns:label locale="">Tags</ns:label>
      </ns:labels>
      <ns:descriptions>
        <ns:description locale="en">A set of tags used to categorise a 
document</ns:description>
        <ns:description locale="">A set of tags used to categorise a 
document</ns:description>
      </ns:descriptions>
      <ns:selectionList>
        <ns:querySelectionList>
          <ns:query>select $Tags where $Tags is not null</ns:query>
          <ns:sortOrder>ascending</ns:sortOrder>
        </ns:querySelectionList>
      </ns:selectionList>
    </ns:fieldType>
  </ns:newFieldType>
</ns:fieldTypeUpdated>