I recently purchased a used, Blond Tornado Coin Op. It is visually in great condition. After further use and inspection, the play surface bows down to the middle. Its not visually noticeable or I would post a picture but it is noticeable when juggling the ball. Its a negligible amount to most, but now that I noticed it its like a thorn in the side. Is there any way to remedy this aside from changing the field? If not, how hard is it to change the field? Please help.