I'm not sure what you mean by updating the grid. If you're having a problem then you may want to contact the support group and provide a sample that demonstrates the problem. You should be able to handle the InitializeFieldLayout event and initialize the FieldSettings there. 

Does nobody have any idea... ?

I tried this method but the grid representation does not update – do you know how I can force the grid to update?

Thank you in advance

 Yes, you can create the same editor style in code and assign it to the FieldSettings' EditorStyle property. Here's the code to create the editor style:

Style style = new Style( typeof( ValueEditor ) );
            style.Setters.Add( new Setter( ValueEditor.FormatProperty, "yyyy/mm/dd" ) );
            style.Seal( );

Is there anyway to do this during run time in the C# code.

 The user preference for how to display Date information is stored on the server and can be different for each user.

How can I do this during run time ?