stepper amperage question

I would like to know if someone out there can shed some light
when you have a unipolar stepper motor and a unipolar stepper
controller the motor is rated at say 5v. and 3. amps if you want to
increase the motor voltage to say 12v. does the amperage also double?
lets say i want to increase the voltage to 24v. what would the
amperage need to be ? is the a formula that someone could tell me?
thanks for the help
kevin