Hi Erekle-Enrico!
You could use XSLT transformation to produce desired result:
<?xml version='1.0'?>
<xsl:stylesheet version="1.0" xmlns:xsl="http://www.w3.org/1999/XSL/Transform"
xmlns:inv2="http://schemas.datacontract.org/2004/07/Invers.Tools.Search.Interface"
xmlns:i="http://www.w3.org/2001/XMLSchema-instance">
<xsl:template match="@*|node()">
<xsl:copy>
<xsl:apply-templates select="@*|node()"/>
</xsl:copy>
</xsl:template>
<xsl:template match="inv2:Value">
<xsl:copy>
<xsl:attribute name="i:type">int</xsl:attribute>
<xsl:apply-templates select="@*|node()"/>
</xsl:copy>
</xsl:template>
</xsl:stylesheet>
It's worth saying that "b:int" construction has no meaning as processor treats it just like a string value of attribute and doesn't add any namespace definitions for it.
Regards, Evgeniy.