update tooltip content when already opened and tip changes.

src/Avalonia.Controls/ToolTip.cs

@@ -55,7 +55,7 @@ namespace Avalonia.Controls
         /// <summary>
         /// Stores the current <see cref="ToolTip"/> instance in the control.
         /// </summary>
-        private static readonly AttachedProperty<ToolTip> ToolTipProperty =
+        internal static readonly AttachedProperty<ToolTip> ToolTipProperty =
             AvaloniaProperty.RegisterAttached<ToolTip, Control, ToolTip>("ToolTip");
 
         private IPopupHost _popup;

src/Avalonia.Controls/ToolTipService.cs

@@ -35,6 +35,13 @@ namespace Avalonia.Controls
                 control.PointerEnter += ControlPointerEnter;
                 control.PointerLeave += ControlPointerLeave;
             }
+
+            if (e.NewValue != e.OldValue && ToolTip.GetIsOpen(control))
+            {
+                var tip = control.GetValue(ToolTip.ToolTipProperty);
+
+                tip.Content = e.NewValue;
+            }
         }
 
         internal void TipOpenChanged(AvaloniaPropertyChangedEventArgs e)