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.
i think it would look alot better with low profile tires! i know my speedometer and odometer would be off, i computed it by 20% running at 60kph would actually be 70kph that would me i should just drive slower i dont think that it would ruin the bearings thou or anything else, cause only difference that i would be facing workst comes to worst would be driving 20% faster which i can easliy fix by driving 20% slower with re gards to abs how would that affect the abs? or tcs?