Go to Wal Mart, pick up a $20 toaster oven, and warm up your lenses slowly. If you don't want to use your paintball goggles, you can use the toaster to form thin sheet acrylic. I've had good luck using a wad of soft cloth to shape lenses for all sorts of different helmets. You just have to be careful not to push hard enough to leave thread marks on the inside of the lenses as you push them out from the inside.
Also, start by baking them at low temperatures to get out any moisture that might be in the plastic. Otherwise, when you get them hot enough to re-shape, you stand a good chance of boiling water making surface bubbles.
It may take a few tries to get this right.
Bear in mind that when you head acrylic (or just about any plastic product) it releases vapors that smell bad, probably harm the environment and your health, and maybe even kill puppies in neighboring houses. Exercise the appropriate precautions.
