Since xsl:copy copies the current element and not the attributes, you need to tell it to copy the attributes.

This will copy any arbitrary attribute:

<xsl:template match="@*">
  <xsl:copy/>
</xsl:template>

So, we can fix the last example to get the attribute copied with the following templates:

<xsl:template match="credit-card">
<xsl:copy>
<xsl:apply-templates select="@*"/>
<xsl:text>XXXX-XXXX-XXXX-XXXX</xsl:text>
</xsl:copy>
</xsl:template>

<xsl:template match="@*">
<xsl:copy/>
</xsl:template>