Here's a photo of the end of a typical 4000 lift arm.
You can pry the ball out past the spring clip and replace it with a cat one ball.
Many implements have both cat one and cat two holes at the top of the 3 point so you can hook it to either size top link. My Ford plow, disc, York rake, Bush Hog and maybe a couple of others do.
I just never noticed the two sizes of holes till I got a cat 2 tractor.