If the outside diameter of the new tires is the same as the old ones
your mileage shouldn't change.  All other things being equal your
handling should improve a bit as there would be less flex in the lower
tire sidewalls.

However at the same time the lower sidewalls will not be able to soak up
shock as well as the taller sidewalls on your current  tires so your
ride will be worse and your wheels and tires will be more susceptible to
damage from impacts.

Jeff DeWitt