TDataset.CopyFields

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 := 0;
  for FieldCtr := 0 to Source.FieldCount - 1 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 > 0 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;