CROFT, MICHAEL wrote:
[snip]
Note that you problem is underspecified, or seriously
misspecified. THerefore, i just included enough to make
it work, without consideration on how it could be
adapted to the problem you really have. Use this on
the data you posted to the list only.

<?xml version="1.0" encoding="ISO-8859-1"?>
<xsl:stylesheet version="1.0" xmlns:xsl="http://www.w3.org/1999/XSL/Transform"
  xmlns:fo="http://www.w3.org/1999/XSL/Format">

  <xsl:key name="AlternateId" match="AlternateId"
    use="parent::KeyGroup/@id"/>

  <xsl:template match="node()|@*">
    <xsl:copy>
      <xsl:apply-templates select="node()|@*"/>
    </xsl:copy>
  </xsl:template>

  <xsl:template match="partyId">
    <xsl:copy>
      <xsl:value-of select="key('AlternateId',../KeyGroup/@refid)[last()]/@value"/>
    </xsl:copy>
  </xsl:template>
</xsl:stylesheet>