I don't think you can change the display because the PropertyGrid general uses TypeConverters (including implicit ones) to display the value, but for multiple selection, it's not used.

What you could do though, but it's not exactly the answer, is propose a custom UITypeEditor when the grid is in multiselection mode, like this:

    public class TestClass
    {
        // decorate the property with a custom UITypeEditor
        [Editor(typeof(MyMultiSelectionEditor), typeof(UITypeEditor))]
        public long TestLong { get; set; }
        public int TestInt { get; set; }
        public TestEnum TestEnum { get; set; }
    }

    public class MyMultiSelectionEditor : UITypeEditor
    {
        public override UITypeEditorEditStyle GetEditStyle(ITypeDescriptorContext context)
        {
            // adapt to your need
            if (!IsPropertyGridInMultiView(context))
                return UITypeEditorEditStyle.None;

            return UITypeEditorEditStyle.Modal;
        }

        public override object EditValue(ITypeDescriptorContext context, IServiceProvider provider, object value)
        {
            if (IsPropertyGridInMultiView(context))
            {
                // multi view, show my custom stuff
                MessageBox.Show("hello from multi selection");
            }
            return base.EditValue(context, provider, value);
        }

        // gets a PropertyGrid instance from the context, if any
        private static PropertyGrid GetPropertyGrid(ITypeDescriptorContext context)
        {
            IServiceProvider sp = context as IServiceProvider;
            if (sp == null)
                return null;

            Control view = sp.GetService(typeof(IWindowsFormsEditorService)) as Control;
            if (view == null)
                return null;

            return view.Parent as PropertyGrid;
        }

        // determines if there is a PropertyGrid in the context, and if it's selection is multiple
        private static bool IsPropertyGridInMultiView(ITypeDescriptorContext context)
        {
            PropertyGrid pg = GetPropertyGrid(context);
            if (pg == null)
                return false;

            return pg.SelectedObjects != null && pg.SelectedObjects.Length > 1;
        }
    }