If you keep the same width and go to a 40 series tire yes you are going to screw a lot of things up.
Your odometer and speedomenter will both be off, you'll wear the bearing and anything else related to the drive shafts faster, your load handling capacity will go down, expect to wear through tires faster, the traction control and abs systems will not work as well, and it'll also look off due to having major wheel well gap unless you lower the car a lot.
I would say why but a better question would be, what are you trying to acheive.