Gear train design

Want to develop a switch over gear train for generating max rpm and max torque for a dc motor
gear ratio for max rpm is 1:2:2.5
gear ratio for max torque is 3:2:1

what are the possible ways ?
one possibility is to reduce voltage which will in-turn reduce rpm to achieve max torque , with use of only one gear ratio 1:2:2.5

is there a switch over mechanism - so i can use 2 different gear trains ? any ideas??