hello Ian,
You will need to look at the content of your definition file so that it deletes the elements you don't want. See the sample-schema.xml.
See the example below, which is code from my definition file for the old ANZLIC ID:
<parameter key="general.ANZLICfileIdentifier" meaning="ANZLICfileIdentifier">
<label resourceKey="catalog.mdParam.general.ANZLICfileIdentifier"/>
<input type="text" size="50" maxlength="2048" defaultValue=""/>
<validation required="false"/> <!-- only required if old ANZLIC id exists -->
<content useSelectForUpdate="true" select="/gmd:MD_Metadata/gmd:identificationInfo/gmd:MD_DataIdentification/gmd:citation/gmd:CI_Citation/gmd:identifier/gmd:MD_Identifier[gmd:authority/gmd:CI_Citation/gmd:title/gco:CharacterString='ANZLIC Identifier']/gmd:code/gco:CharacterString"
delete="/gmd:MD_Metadata/gmd:identificationInfo/gmd:MD_DataIdentification/gmd:citation/gmd:CI_Citation/gmd:identifier"/>
</parameter>
The above code will delete all elements under gmd:identifier if the field ../gmd:identifier/gmd:MD_Identifier/gmd:code/gco:CharacterString is not completed. This also has a qualifier the field under gmd:identifier "/gmd:MD_Identifier[gmd:authority/gmd:CI_Citation/gmd:title/gco:CharacterString='ANZLIC Identifier']" ie: The gmd:identifier elements must be for ANZLIC Identifiers. Hope that makes sense see W3Schools for further insight into the XML useage.
Warwick