Dave,
You'll need to put the logic as to when to overwrite and when to add into
some xslt, however, if you read the DirXML DTD you'll see that if DirXML
recieves a <remove-value> element or a <remove-all-values/> element
inside the <modify> element then it will overwrite. For example This will
replace all values with a new value
<modify class-name="User">
<association
state="associated">12345</association>
<modify-attr attr-name="Description">
<remove-all-values/>
<add-value>
<value
type="string">this is the new value</value>
</add-value>
</modify-attr>
</modify>
This will remove a specific value and add another
<modify class-name="User">
<association
state="associated">12345</association>
<modify-attr attr-name="Description">
<remove-value>
<value type="string">this is the old value</value>
</remove-value>
<add-value>
<value
type="string">this is the new value</value>
</add-value>
</modify-attr>
</modify>
Hope this helps,