combining xml trees

I have an xml tree that looks like this:

<root>
    <xxx>
       <yyy>
          <.../>
       </yyy>
       <zzz>
          <.../>
       </zzz>
    </xxx>
</root>

The base tree is stored in a file on the local computer. 
Periodically, 
the local computer queries another machine on the network,
and receives 
a partial update to the main xml tree, say something like
this:

<root>
    <xxx>
       <zzz>
          <.../>
       </zzz>
    </xxx>
</root>

The partial tree will not have any new nodes; just new data
for already 
existing nodes.

Is there some clean way to fold the new tree into the old
tree without 
losing any of the other data?

In the above example, I don't want to lose <root>
<xxx> <yyy> but need 
to replace <zzz>.

Thanks,

--Yan
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