I used a stainless steel carpenter's rule, you know the kind with a sliding 90 degree edge and level on it? It was a couple of inches wide and about 1/8" thick. I sharpened one end with a file. I drove the sharp end down both flat sides of each chainplate, between the chainplate and the deck. Using a block of wood and a pry bar through the hole in the top of the chainplate, I was able to pry the chainplate up, breaking the sealant.
Total time to remove all eight chainplates was four hours.
Several of the chainplates showed signs of crevice corrosion. Dan of PT Rigging ground down the cracks and re-welded the chainplates. A little grinding and polishing and the chainplates were like new!