If you do need to replace any of the MOSFETS I suggest that you use one with a lower Ron resistance like an IRLB3036PBF, http://www.mouser.com/ProductDetail/International-Rectifier/IRLB3036PBF/?qs=9%252bKlkBgLFf2IRFFZoHru3w%3D%3D. A few dollars more but will run cooler and be able to keep any heated bed hotter.
Also be careful when removing the old ones. It is easy to overheat and pull up the solder pads. Be sure to cut out the old one first before trying to desolder.