これを試して:
<?xml version="1.0"?>
<!-- Transform a document to itself, lowercasing all tag names -->
<xsl:stylesheet version="1.0" xmlns:xsl="http://www.w3.org/1999/XSL/Transform">
<!-- Import the identity transformation -->
<!-- Whenever you match any node or any attribute -->
<xsl:template match="node()|@*">
<!-- Copy the current node -->
<xsl:copy>
<!-- Including any attributes it has and any child nodes -->
<xsl:apply-templates select="@*|node()"/>
</xsl:copy>
</xsl:template>
<!-- Whenever you match any node or any attribute -->
<!-- When you match any element -->
<xsl:template match="*">
<!-- Create the same element with a lowercase name -->
<xsl:element name="{translate(name(),'ABCDEFGHIJKLMNOPQRSTUVWXYZ', 'abcdefghijklmnopqrstuvwxyz')}">
<!-- Including any attributes it has and any child nodes -->
<xsl:apply-templates select="@*|node()"/>
</xsl:element>
</xsl:template>
</xsl:stylesheet>