Presumably you are aware that you can't just replace the servos with steppers?, either electrically or mechanically - servos are absolute devices (you tell it to go to point A, and it goes there and stays there). Steppers are 'relative', you tell it to go so many steps from where it currently is, and in what direction, and it will attempt to do so - but you've no idea if it actually made it or not. A servo will get there, or at least keep trying to do so - a stepper will simply make an attempt, and has no idea if it succeeded or not.
I'm presuming you're just wanting to use the steppers on their own, with no additional gearing?, and no kind of external positioning feedback - it's likely to considerably inferior to servos in that case.