The majority of the differences of V5 is under the hood. Expressions, joints for bending, shape morphs, etc. have been tweaked for V5. So if you were to apply a pose or expression specifically for V5 to base genesis, the bend or pose will be off. Also if you use the body dials from Evolution morphs while the V5 shape is active,  it will actually be calling the tweaked version of the morph for V5, not the base genesis one.