I am not sure but I have a feeling that Michael's solution is
overkill. Maybe a solution as simple as the following is enough, but
as Michael says:

"you haven't really given any clues as to how much variability you
want to handle."

<?xml version="1.0" encoding="UTF-8"?>
<xsl:stylesheet xmlns:xsl="http://www.w3.org/1999/XSL/Transform" version="2.0">
    <xsl:output indent="yes"/>
    <xsl:template match="/">
    <Root>
        <RandomTag1><xsl:value-of select="Root/RandomTag1"/></RandomTag1>
        <RC>
            <xsl:for-each select="Root/*[starts-with(name(), 'RC')]">
                <xsl:element name="{substring-after(name(),
'RC')}"><xsl:value-of select="."/></xsl:element>
            </xsl:for-each>
        </RC>
        <FG>
            <xsl:for-each select="Root/*[starts-with(name(), 'FG')]">
                <xsl:element name="{substring-after(name(),
'FG')}"><xsl:value-of select="."/></xsl:element>
            </xsl:for-each>
        </FG>
        <RandomTag2><xsl:value-of select="Root/RandomTag2"/></RandomTag2>
        <RandomTag3><xsl:value-of select="Root/RandomTag3"/></RandomTag3>
    </Root>
</xsl:template>
</xsl:stylesheet>

Cheers,
Jesper Tverskov
http://www.xmlplease.com