XML : Error: The target of 'replace value of' must be a non-metadata attribute or an element with simple typed content

I have the following xml field in which I need to replace the value of one of the nodes:

  <DataFormItem xmlns:xsd="http://www.w3.org/2001/XMLSchema" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns="bb_appfx_dataforms">    <Values>      <fv ID="FUNDRAISERID">        <Value xmlns:q1="http://microsoft.com/wsdl/types/" xsi:type="q1:guid">1009fb5a-41a0-40c1-8685-d5973fb71df7</Value>        <ValueTranslation>John Smith</ValueTranslation>      </fv>      <fv ID="STARTDATE">        <Value xsi:type="xsd:dateTime">2015-01-01T00:00:00</Value>      </fv>    </Values>  </DataFormItem>    

I need to change the STARTDATE value '2015-01-01T00:00:00' to '2016-01-01T00:00:00'. To accomplish this, I have written the following query:

  update KPIINSTANCE  SET     NAME = '2016 ' + SUBSTRING(k.name, 6,150),           PARAMETERSXML.modify('          declare namespace df = "bb_appfx_dataforms";          replace value of (/df:DataFormItem/df:Values/df:STARTDATE/df:Value)[1] with "2016-01-01T00:00:00" cast as xs:dateTime ?')  FROM KPIINSTANCE K          JOIN KPICATALOG KP ON KP.ID = K.KPICATALOGID  where k.ID = '43C6DA6B-420A-4D4E-BA31-84C8054B4AB6'    

As far as I can tell, this is correct, but when I try to run the query, I get the error: XQuery [KPIINSTANCE.PARAMETERSXML.modify()]: The target of 'replace value of' must be a non-metadata attribute or an element with simple typed content, found 'element(df{bb_appfx_dataforms}:Value,xdt:untyped) ?'

Since this node is typed, I'm unsure why the error is saying it is not typed. Anyone have any ideas?