function TDataSet.CopyFields(Source: TDataSet): Integer;

// copies matching fields in current records- returns number of fields copied

var

  FieldCtr: Integer;

  DestField, SourceField: TField;

begin

  Result := ;

  for FieldCtr :=  to Source.FieldCount -  do begin

    SourceField := Source.Fields[FieldCtr];

    Field := FindField(SourceField.FieldName);

    if not Assigned(Field) then Continue;

    if Field.ClassType = TDataSetField then begin  // nested datasets

      while TDataSetField(Field).NestedDataSet.RecordCount >  do

        TDataSetField(Field).NestedDataSet.Delete;

      TDataSetField(SourceField).NestedDataSet.First;

      while not TDataSetField(SourceField).NestedDataSet.Eof do begin

        TDataSetField(Field).NestedDataSet.Append;

        CopyFields(TDataSetField(Field).NestedDataSet, TDataSetField(SourceField).NestedDataSet);

        TDataSetField(Field).NestedDataSet.Post;

        TDataSetField(SourceField).NestedDataSet.Next;

      end;

    end else

      Field.Value := SourceField.Value;

    Inc(Result);

  end;

end;

This would typically be used as follows:

SourceDS.First;

while not SourceDS.EOF do begin

  if DestDS.Locate({info required to find matching record}) then begin

    DestDS.Edit;

    DestDS.CopyFields(SourceDS);

    DestDS.Post;

  end;

  SourceDS.Next;

end;