Replace existing element with another from an external XML - xslt

Question

I would like to replace an element from an external XML file. Currently I have an XSLT which adds elements, but not replacing them.
Input.xml:
<all>
<item>
<sku>3850950</sku>
<name>Macbook Pro</name>
<qty>3</qty>
<img>original.jpg</img>
</item>
<item>
<sku>3859955</sku>
<name>iPhone 12</name>
<qty>9</qty>
<img>original-apple.jpg</img>
</item>
</all>
XSLT:
<xsl:stylesheet version="2.0"
xmlns:xsl="http://www.w3.org/1999/XSL/Transform">
<xsl:output method="xml" version="1.0" encoding="UTF-8" indent="yes"/>
<xsl:template match="node()|#*">
<xsl:copy>
<xsl:apply-templates select="node()|#*"/>
</xsl:copy>
</xsl:template>
<xsl:key name="group" match="additem" use="sku" />
<xsl:template match="item">
<xsl:copy>
<xsl:copy-of select="*"/>
<xsl:apply-templates select="key('group', sku, document('./extra-info.xml'))"/>
</xsl:copy>
</xsl:template>
<xsl:template match="additem">
<xsl:for-each select="img">
<img>
<xsl:value-of select="."/>
</img>
</xsl:for-each>
</xsl:template>
</xsl:stylesheet>
Extra-info.xml:
<all>
<additem>
<sku>3850950</sku>
<img>aaaaaaa</img>
<img>bbbbbbb</img>
</additem>
<additem>
<sku>3859955</sku>
</additem>
</all>
I would like to match img elements from extra-info.xml, the common key is sku. Replace <img> in the input.xml if same exists in extra-info.xml and add all of them.
Example output:
<all>
<item>
<sku>3850950</sku>
<name>Macbook Pro</name>
<qty>3</qty>
<img>aaaaaaa</img>
<img>bbbbbbb</img>
</item>
<item>
<sku>3859955</sku>
<name>iPhone 12</name>
<qty>9</qty>
<img>original-apple.jpg</img>
</item>
</all>

Answer 1

AFAICT, you want to do:
XSLT 2.0
<xsl:stylesheet version="2.0"
xmlns:xsl="http://www.w3.org/1999/XSL/Transform">
<xsl:output method="xml" version="1.0" encoding="UTF-8" indent="yes"/>
<xsl:key name="additem" match="additem" use="sku" />
<!-- identity transform -->
<xsl:template match="#*|node()">
<xsl:copy>
<xsl:apply-templates select="#*|node()"/>
</xsl:copy>
</xsl:template>
<xsl:template match="item">
<xsl:variable name="additem" select="key('additem', sku, document('extra-info.xml'))"/>
<xsl:copy>
<xsl:copy-of select="* except img"/>
<xsl:choose>
<xsl:when test="$additem/img">
<xsl:copy-of select="$additem/img"/>
</xsl:when>
<xsl:otherwise>
<xsl:copy-of select="img"/>
</xsl:otherwise>
</xsl:choose>
</xsl:copy>
</xsl:template>
</xsl:stylesheet>