I believe that they are different sized heads and spring bars and are not interchangeable. Equal-i-zer used to have the physical sizes of the spring bars and weights of the products on their website but that is no longer there. What they have now is a PDF of the parts breakdown
here..
Looking at that parts breakdown, it shows different part numbers for the hitch head of every weight hitch. 6, 10, 12, and 14 thousand pound hitch heads and bars all have a different part number. All the other part numbers are identical. That tells me that the parts are still not interchangeable, just as it used to be. If you want to upgrade to the next higher weight class, it looks to me like you will need to purchase a whole new hitch. The only parts that you could keep would be the parts that attach to the trailer A frame (L brackets and parts)and probably your shank and ball.
Barney