Check out
Mazda G4A-EL transmission - Wikipedia, the free encyclopedia that should be your trans it is also known as the Ford 4EAT-G.
For more info check out
Mazda626.net Forums they should know if it is possible.
But yes it is possible to change the gearing on a FWD auto trans, on your car I'm not sure. On the gm 4t65-E there are several ones from 2.97 to 3.62 but that is only the final drive I do not know of anyone changing the individual gears.