Something along these lines, if I understand you correctly:
<xsl:element name="name()">
<xsl:value-of select="./text()" />
</xsl:element>
This XSL Stylesheet:
<xsl:stylesheet version="1.0" xmlns:xsl="http://www.w3.org/1999/XSL/Transform">
<xsl:output indent="yes"/>
<xsl:strip-space elements="*"/>
<xsl:template match="node()|@*">
<xsl:copy>
<xsl:apply-templates select="node()|@*"/>
</xsl:copy>
</xsl:template>
<xsl:template match="Field">
<xsl:element name="{@Name}">
<xsl:value-of select="@Value"/>
</xsl:element>
</xsl:template>
</xsl:stylesheet>
Applied to well-formed input:
<SiebelMessage MessageId="1-18J35" IntObjectName="XRX R5 Letter Instance" MessageType="Integration Object" IntObjectFormat="Siebel Hierarchical">
<LetterInstance Id="1-1RUYIF" Language="ENU" TemplateType="SA">
<Field Value="CO Last Name" Datatype="String" Name="ContractingOfficerLastName"/>
</LetterInstance>
</SiebelMessage>
Produces:
<SiebelMessage MessageId="1-18J35" IntObjectName="XRX R5 Letter Instance" MessageType="Integration Object" IntObjectFormat="Siebel Hierarchical">
<LetterInstance Id="1-1RUYIF" Language="ENU" TemplateType="SA">
<ContractingOfficerLastName>CO Last Name</ContractingOfficerLastName>
</LetterInstance>
</SiebelMessage>
I'm not sure where <PONumber>
is supposed to be generated from.