Xslt: How to change node text in element using a parameter

I need to change the text of certain nodes, similar to Update the text of an element with XSLT based on param.

This is my XML:

<entry>
    <form type="hyperlemma" xml:lang="cu">
         <orth>абиѥ</orth>
    </form>
    <form type="lemma" xml:lang="cu">
         <orth>абиѥ</orth>      
    </form>
    <form type="variant" xml:lang="cu">
         <orth>а̓бїе</orth>
         <form type="hyperlemma" xml:lang="cu">
             <orth>а̓бїе</orth>
         </form>
    </form>
</entry>

I now want to replace the text between <orth> in

<form type="variant" xml:lang="cu">
    <orth>а̓бїе</orth>
    <form type="hyperlemma" xml:lang="cu">
        <orth>а̓бїе</orth>
    </form>
</form>

by the content of <orth> in the previous node

<entry>
    <form type="hyperlemma" xml:lang="cu">
        <orth>абиѥ</orth>
    </form>

in order to get the following output:

<entry>
    <form type="hyperlemma" xml:lang="cu">
        <orth>абиѥ</orth>
    </form>
    <form type="lemma" xml:lang="cu">
        <orth>абиѥ</orth>      
    </form>
    <form type="variant" xml:lang="cu">
        <orth>а̓бїе</orth>
        <form type="hyperlemma" xml:lang="cu">
            <orth>абиѥ</orth>
        </form>
    </form>
 <entry>

When I use the following stylesheet

<?xml version="1.0" encoding="UTF-8"?>
<xsl:stylesheet xmlns:xsl="http://ift.tt/tCZ8VR" xmlns:xs="http://ift.tt/tphNwY" exclude-result-prefixes="xs" xpath-default-namespace="http://ift.tt/1dDT9n4" version="2.0">
<xsl:output method="xml" indent="yes" />
<xsl:strip-space elements="*" />

<xsl:param name="replace_orth" select="entry/form[@type='hyperlemma' and @xml:lang='cu']/orth" />

<xsl:template match="node()|@*">
    <xsl:copy>
        <xsl:apply-templates select="node()|@*" />
    </xsl:copy>
</xsl:template>

<xsl:template match="form[@type='variant']/form[@type='hyperlemma' and @xml:lang='cu']/orth/text()">
    <xsl:value-of select="$replace_orth" />
</xsl:template>
</xsl:stylesheet>

then I get

<form type="variant" xml:lang="cu">
    <orth>а̓бїе</orth>
    <form type="hyperlemma" xml:lang="cu">
         <orth/>
</form>

So <orth> is empty. If I change the parameter to

<xsl:param name="replace_orth" select="'new orth'" />

'new orth' gets printed. But as the content of <entry><form type="hyperlemma" xml:lang="cu"><orth> is different for each entry, I cannot use a 'static' string.

What do I need to change?

Thanks for any hint!