On some basic Mod(20") bikes, it has 20" wheels front and rear, so the rear magura mounts will be higher up on the frame to suit the larger wheel, so a 19" won't fit- so it's hard to upgrade frames.
On most mod bikes, the rear wheel and brake placement is suited to 19" and is, therefore, easier to change frames as all modern frames have brake mounts to suit a 19" wheel.
Hope that's shed some light.