You don't necessarily need to buy new servos. But to make your quad walk I would suggest to remove the 4. DOF (the servos inside the body), that would also save some weight. Then make a new upper and lower body section for supporting the coxa servo on both sides. Decrease the femur length as much as possible, maybe 50-40 mm length, its hard to say without not knowing the exact weight of the robot. Also, analyze every part/bracket to see if you can decrease the weight, e.g. drill some holes on the tibia, reduce the length on every screw that holds the servos. Use an accurate weight scale for measuring the main parts before and after doing the modifications for reducing the weight.