Can you have more than just b and i elements? It may be possible to do this with a generic solution, that creates a nested element for each child element of a span element.
This solution uses a recursive template, that matches span, but with a parameter contain the index number of the child element that needs to be output. When this index exceeds the number of child elements, the text is output.
Try this XSLT too:
<xsl:stylesheet version="1.0" xmlns:xsl="http://www.w3.org/1999/XSL/Transform">
<xsl:output method="xml" indent="yes"/>
<xsl:template match="@*|node()">
<xsl:copy>
<xsl:apply-templates select="@*|node()"/>
</xsl:copy>
</xsl:template>
<xsl:template match="span">
<xsl:param name="num" select="1"/>
<xsl:variable name="childElement" select="*[$num]"/>
<xsl:choose>
<xsl:when test="$childElement">
<xsl:element name="{local-name($childElement)}">
<xsl:apply-templates select=".">
<xsl:with-param name="num" select="$num + 1"/>
</xsl:apply-templates>
</xsl:element>
</xsl:when>
<xsl:otherwise>
<xsl:value-of select="."/>
</xsl:otherwise>
</xsl:choose>
</xsl:template>
</xsl:stylesheet>
This does assume that all the span element only contain elements you want to nest, in addition to the text.