How to reset xsl parameter values?

Good morning,

I am new to XML and XSL.  I am currently working on an XSL stylesheet and I need to know how to use a parameter locally and reset the values for use later.  Here is an example of what I'm trying to do:

<xsl:param name="querystring">not set yet</xsl:param>

<xsl:choose>
   <xsl:when test="@file_type='File'">
      <!--I want to set the "querystring" variable to a specific text value -->
   </xsl:when>
   <xsl:when test="@file_type='Date'">
      <!--I want to set the "querystring" variable to a specific text value-->
   </xsl:when>
   <xsl:otherwise>
      <!--I want to set the "querystring" variable to a specific text value-->
   </xsl:otherwise>
</xsl:choose>

<!--Then I want to use this variable to populate the HTML cell -->
<td><xsl:value-of select ="{$querystring}"/></td>  <!--Is that right?-->

Thanks