It wouldn't cause havoc, it would just shift differently. It calculates when to shift based on RPM, current speed, and gas pedal positions. If it does suck somehow, you can always go into steptronic mode. The automatics don't downshift until the RPMs are somewhere below 1,000 so they wont accidentally over-rev unless you go into steptronic and it's lower gear RPM prediction is too low due to the ratiochange, so I guess just be careful when doing that.
I heard from someone at a BMW dealership that they reset my "computer" and my transmission will "re-learn my driving style" when it was in for some reapairs, so it may very well automatically detect each gear's sped limits based on whatever gearing ratio it's hooked up to.