You want to transform the xml using xsl/xslt and perform the identity transform on all data except for the data you want to change. Add an xslt rule for the data you want changed and (duh) make the change there.
The identity transform looks like:
<xsl:stylesheet version="1.0" xmlns:xsl="http://www.w3.org/1999/XSL/Transform">
<xsl:template match="@*|*|processing-instruction()|comment()">
<xsl:copy>
<xsl:apply-templates select="@*|*|text()|processing-instruction()|comment()"/>
</xsl:copy>
</xsl:template>
</xsl:stylesheet>
[The only things this transform will not preserve are formatting, white space or CDATA but that does not seem to be an issue for the data you present here.]