I just redid mine. In general finding good canvas people on the Eastern Shore is a challenge. The tops got done just fine. The bottom pads the guy didn't get the canvas tight enough, and he didn't put the seam across the middle, so it tends to sail as the wind blows over it. He says he'll fix but I haven't had the time to take them back.
So to answer the question the top backrests can be done, but YMMV may vary on the bottom part.