Or alternatively you could turn it on its head and have mod_quiz extend quiz. The new overridden methods call the parent methods with an option not to display the depreciated message (as downcasting works). Place 'depreciated' message in the 'quiz' methods so it gets printed when the old reference is used to call the methods with the old parameter declarations. Then if the theme renderers call the methods with the old parameter object with a returned mod_quiz reference then it will still work. But you will use the methods with the new parameter objects. And then strip out the quiz layer in M2.7 and implement all of the code.
I think I may need to create an example script.