Performance issue with nested TClientDataSet

Question

I have an application which uses client datasets as in memory tables, and local archive for data received from laboratory instruments. Over time, I've added a few fields to these client datasets, and in order to keep the saved data accessible, I've written a small procedure which literally updates the old saved files. So far it has been working perfectly. Recently, I had to use nested datasets, and I tried to change my code to iterate through nested datasets and update them as well. The code I've come up with so far is this:

procedure UpdateCDS(NewCDS: TClientDataSet; const OldFileName: string);
  procedure InternalUpdateCDS(NewCDS, OldCDS: TClientDataSet);
  var
    i, j: Integer;
    NewNestedCDS, OldNestedCDS: TClientDataSet;
  begin
    OldCDS.First;
    while not OldCDS.Eof do
    begin
      NewCDS.Append;
      for i := 0 to NewCDS.FieldDefs.Count - 1 do
        if NewCDS.Fields[i].FieldKind = fkData then
          for j := 0 to OldCDS.Fields.Count - 1 do
            if UpperCase(NewCDS.Fields[i].FieldName)
              = UpperCase(OldCDS.Fields[j].FieldName) then
            begin
              if not OldCDS.Fields[j].IsNull then
                if OldCDS.Fields[j].DataType <> ftDataSet then
                  NewCDS.Fields[i].Value := OldCDS.Fields[j].Value
                else
                begin
                  NewNestedCDS := TClientDataSet.Create(nil);
                  try
                    OldNestedCDS := TClientDataSet.Create(nil);
                    try
                      OldNestedCDS.DataSetField :=
                        TDataSetField(OldCDS.Fields[j]);
                      NewNestedCDS.DataSetField :=
                        TDataSetField(NewCDS.Fields[i]);
                      OldNestedCDS.Open;
                      NewNestedCDS.Open;
                      InternalUpdateCDS(NewNestedCDS, OldNestedCDS);
                    finally
                      OldNestedCDS.Free;
                    end;
                  finally
                    NewNestedCDS.Free;
                  end;
                end;
              Break;
            end;
      NewCDS.Post;
      OldCDS.Next;
    end;
    NewCDS.MergeChangeLog;
  end;

var
  OldCDS: TClientDataSet;
begin
  OldCDS := TClientDataSet.Create(nil);
  try
    OldCDS.LoadFromFile(OldFileName);
    InternalUpdateCDS(NewCDS, OldCDS);
  finally
    OldCDS.Free;
  end;
end;

My problem is, this code is way too slow. For 1000 records, it takes about 10 seconds on my intel T7500. Is there a faster way to update a client dataset?