Use the modified identity template:
<xsl:template match="client:*" xmlns:client="http://client1">
<xsl:element name="{name()}" namespace="http://client2">
<xsl:copy-of select="@*"/>
<xsl:apply-templates/>
</xsl:element>
</xsl:template>
XSLT 1.0 encourages processors to use the prefix given by "name()", that is the original prefix. XSLT 2.0 makes it mandatory to use this prefix unless there is a conflict.
Michael Kay
http://www.saxonica.com/
Author, XSLT Programmer's Reference and XPath 2.0 Programmer's Reference