You may or may not have rear sensors. Usually the front pads are what makes the light come on, not always of course, but the front brakes wear out at about twice the rate of the rears.
The hazard of not replacing the pads, other than the possibility of wearing the pad surface off completely, is that if you do wear the pad surface off completely, you could ruin the rotors, which may otherwise be able to be reused. You need to measure the thickness of the rotors and see if they are ablove the service spec (this isn't the same as the min. thickness, the service spec is usually around .5-.6mm thicker than the min. spec. . DON'T machine the rotors, either they are reuseable or they're not.
I recommend against the transmission service. If the transmission fails it won't be because of the fluid.