The problem with animating the content is that the selected content in the TabControl is changed right after the menu is selected and there is never a second object that you can animate.
One possibility here is to use a WritableBitmap in which to draw the old content and then animate this WritableBitmap. This though will not be trivial because it will most probably involve inheriting the TabControl and changing its ControlTemplate to add the Image and plug in the logic with the WritableBitmap.
In short - it is possible but it will involve a lot of custom code.
the Telerik team