I've had 2 pairs of Bates side zip for years. The first pair zippers finally started to slip down. I took them to a local cobbler and he suggested sewing on a piece of 'tuff' velcro on the upper side to cover the zipper head. Works great. The velcro keeps the zipper from sliding down.
Its hard to go back to non-side zip boots once you get used to the easy-on easy-off.
Its hard to go back to non-side zip boots once you get used to the easy-on easy-off.