I don't think you can change the node types (from the child of type BBASE to BDERIVED) in the scene tree of an inherited scene.
Depending on your goal, you might be able to achieve what you want to do by using script inheritance instead of scenes, and dynamically remove and add scripts of the prefered type.
Also, what you could do is have a scene ABASE, then define two inherited scenes, ADERIVED1 that has the child scene BBASE, and ADERIVED2 that has the child scene BDERIVED. Although this proposition might not meet your requirements.