the transition between the different music styles (together with graphics) is a great idea (but imo applied too often), I will try to do something similar when I step to game music creation stage (it is not too complicated with midi based instruments, or when e.g. guitar is recorded dry, and only software effects are used, I have some experience in it.)
, to be honest, I dont like transitions between the music in Giana very much. Just like battle music in games like UT 3, it changes pace too often. Maybe you can do in your game that music changes but it loops out and loops in (through each other, so that the change is less noticeable and more subtle). I find these visual transitions adorable though.
