Theoretically, if your new frame is smaller than the old one or exactly the same as the old one, you can regrind the old lenses to fit the new frames. Moreover, the shape of the frame should exactly the same or approximately the same. That is to say, you can not regrind a round lenses to fit a rectangle frame. Moreover, you also need to considerate the possibility that your lenses will be destroyed during the processing of regrinding. So, the best option is to buy a new pair of eyeglasses.