I haven't opened up one of our servos, but most use a potentiometer linked to the motor's gear train to determine their angular position. "Hacking" a standard servo into a continuous rotation servo typically requires that this potentiometer be held to the middle value (or removed in favor of a fixed voltage divider) and that the mechanical stops inside the servo be removed. These stops are usually nothing more than "extra" bits of plastic inside the housing and on the gears themselves that restrict the horn's movement.
The hack is pretty straightforward (it'll require a little soldering), but if I were you I'd buy CR servos; you're likely to have more consistent, reliable results. In my experience, homebrew CR servos tend to drift (an artifact of how the control circuitry works when the potentiometer is not perfectly centered).