dirty.xml

<?xml version="1.0" encoding="UTF-8"?>
<root>
  <url>See http://www.uis.no</url>
</root>

schema.xsd

<?xml version="1.0" encoding="UTF-8"?>
<xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema" elementFormDefault="qualified"
  targetNamespace="http://fsws.usit.no/schemas/studinfo"
  xmlns:studinfo="http://fsws.usit.no/schemas/studinfo">

  <xs:element name="root">
    <xs:complexType>
      <xs:sequence>
        <xs:element name="url" minOccurs="0" type="xs:anyURI"/>
        <xs:element name="url-text" minOccurs="0" type="xs:string"/>
      </xs:sequence>
    </xs:complexType>
  </xs:element>
</xs:schema>

transform.xsl

<?xml version="1.0" encoding="UTF-8"?>
<xsl:stylesheet xmlns:xsl="http://www.w3.org/1999/XSL/Transform"
  xmlns:xs="http://www.w3.org/2001/XMLSchema"
  exclude-result-prefixes="xs"
  version="2.0">

  <xsl:template match="*|@*">
    <xsl:copy>
      <xsl:apply-templates select="node()|@*"/>
    </xsl:copy>
  </xsl:template>
    
  <xsl:template match="url">
    <xsl:call-template name="dirty_URL">
      <xsl:with-param name="add-to-element" select="'url-text'"/>
    </xsl:call-template>
  </xsl:template>
    
  <xsl:template name="dirty_URL">
    <xsl:param name="add-to-element"/>
    <xsl:variable name="url">
      <xsl:apply-templates select="text()" mode="serialize"/>
    </xsl:variable> 
    <xsl:variable name="new_url">
      <xsl:value-of select="iri-to-uri($url)"/>
    </xsl:variable>
    <xsl:choose>
      <xsl:when test="$url = $new_url">
        <xsl:copy>
          <xsl:value-of select="$new_url"/>
        </xsl:copy>
      </xsl:when>
      <xsl:otherwise>
        <xsl:element name="{$add-to-element}">
          <xsl:value-of select="text()"/>
        </xsl:element>
      </xsl:otherwise>
    </xsl:choose>
  </xsl:template>

  <xsl:template match="*" mode="serialize">
    <xsl:text>&lt;</xsl:text>
    <xsl:value-of select="name()"/>
    <xsl:text>&gt;</xsl:text>
    <xsl:apply-templates mode="serialize"/>
    <xsl:text>&lt;/</xsl:text>
    <xsl:value-of select="name()"/>
    <xsl:text>&gt;</xsl:text>
  </xsl:template>
    
  <xsl:template match="text()" mode="serialize">
    <xsl:value-of select="." xml:space="preserve"/>
  </xsl:template>

</xsl:stylesheet>