Well, I've found the problem. Apparently, the terminal on the spark plug is threaded, and mounts on a screw on the top of the plug. The plug was impossible to pull off because this terminal was frozen somehow to the plug wire boot; I got it off by twisting the terminal off the spark plug with the boot. This is not a good situation; the plug wire boot won't mate to the new plug because it still has the old plug's terminal inside.

What in the world could cause these terminals to become so completely stuck together? Anyhow, since time is in short supply, I think I will replace the old plug, and screw the boot and corresponding terminal back onto the old plug, so I can get it to the shop for new wires.