The esc should say what voltage it operates at. Make sure it matches what your setup is designed for. I am not sure about this, but a point to check before you do any modifications. A typical spec writeup for ESC's is shown in the below link from Tower Hobbies in the US. It shows the battery range that will work with this particluar ESC. Make sure the one you decide on will give you similar performance (multiple battery, current rating, and so on).
http://www3.towerhobbies.com/cgi-bin...&I=LXKSY6&P=ML