How to change <xsl:variable> value

I can't get this to work... I simply want to change the value of a globally defined variable:

                    <xsl:variable name="isBusiness"></xsl:variable>
                    <xsl:choose>
                        <xsl:when test="yes this is a business">
                               <xsl:variable name="isBusiness">true</xsl:variable>                        
                        </xsl:when>
                        <xsl:otherwise>
                            <xsl:variable name="isBusiness">false</xsl:variable>
                        </xsl:otherwise>
                    </xsl:choose>    

Obviously the code is invalid because is already defined, but how would I change the value?