How to add a field to a TClientDataSet in an existing app?

Question

In an existing app, a TClientDataSet "Band.CDS" has 3 fields defined using FieldDefs: "Name", "Genre" and "URL". Now want to add a 4th field.

At launch, FormCreate does this:

  appPath := ExtractFilePath(Application.ExeName);
  BandCDS.FileName := appPath + 'bands.xml'; // an existing file
  BandCDS.Active := TRUE;
  BandCDS.LoadFromFile(BandCDS.FileName);  // load from a file

A TDataSource points to BandCDS, and a TDBGrid points to that TDataSource.

Everything works fine. But I now want to add a 4th field to the TClientDataSet and have it shown to the user (along with the other fields) in the TDBGrid.

What does NOT work: simply adding a fourth item to the FieldDefs for BandCDS.

ADDED INFORMATION ---

At launch, The CDS is loaded from an XML file:

appPath := ExtractFilePath(Application.ExeName);
  BandCDS.FileName := appPath + 'bands.xml';

At the end of the session, the CDS is saved back out to the same file:

BandCDS.SaveToFile(BandCDS.FileName, dfXML);

After the file is loaded, I tried changing a field name and adding a new field:

BandCDS.FieldDefs[1].Name := 'NewName';
BandCDS.FieldDefs.Add('A',ftFixedChar,1);

• After that when I examine the FieldDefs, the name HAS been changed, and the new field HAS been added
• However, the DBGrid display is unchanged.
• If I save the file (as mentioned above), exit, and re-launch, we're back to the original names and fields.
• Clearly, the field revisions didn't affect what was output by the SaveToFile.

Thank you! Kevin