update/re-add an existing document with numeric fields

[Tim Eck]

Note: I'm bound to lucene 3.0.3 for the context of this question, but 
I would be interested to know if newer versions would help me here.  
 
I have an existing document in my directory that has one regular
String field and one numeric field. I naively thought I could update
that document to change the String field with code like this: 
  
  FSDirectory dir = FSDirectory.open(...);
  IndexWriter writer = new IndexWriter(dir, new
      StandardAnalyzer(Version.LUCENE_30), MaxFieldLength.UNLIMITED);
 
  // doc has 2 fields, one String and the other numeric
  Document doc = new Document();
  doc.add(new Field("string", "value", Store.YES,
      Index.ANALYZED_NO_NORMS)); 
  NumericField nf = new NumericField("numeric", Field.Store.YES,
      true); 
  nf.setIntValue(42);
  doc.add(nf);
  writer.addDocument(doc);
  writer.commit();
 
  // make sure we can query on the numeric field
  IndexSearcher searcher = new IndexSearcher(dir);
  TopDocs docs = searcher.search(new TermQuery(new Term("numeric",
      NumericUtils.intToPrefixCoded(42))), 1); 
  if (docs.totalHits != 1) {
      throw new AssertionError();
  }
  doc = searcher.doc(docs.scoreDocs[0].doc);
  searcher.close();
 
  // update document with new value for string field
  doc.removeField("string");
  doc.add(new Field("string", "value2", Store.YES,
      Index.ANALYZED_NO_NORMS)); 
  writer.updateDocument(new Term("string", "value"), doc);
  writer.commit();
 
  // search again
  searcher = new IndexSearcher(dir);
  docs = searcher.search(new TermQuery(new Term("numeric",
      NumericUtils.intToPrefixCoded(42))), 1); 
  if (docs.totalHits != 1) {
      throw new AssertionError(docs.totalHits);
  }
 
 
That doesn't seem to work however. It seems I need to get the 
NumericField rematerialized in the document passed to
updateDocument(). I was hoping to avoid that if possible so
I'm looking for any suggestions someone might offer.