The main issue is that you'll need a different type of control circuit - servos are driven with a timing pulse that typically ranges from 1.0 - 2.0 milliseconds every 20 milliseconds, while DC motors are driven by a continuous pulse stream called Pulse Width Modulation (PWM) with a varying duty cycle (0 - 100%). You'll need an h-bridge or a set of MOSFET's to switch the current going to the motors, while no interface circuit is generally required for servos.
There might already be a tutorial on this site about interfacing to servos vs DC motors.