I'm not sure if the subtleties of the question have been answered. Its one I've never thought of and don't know the answer to.

On a 2 class AA flight there is A and M inventory but is there D inventory - I don't think there is. So if you buy an extra segment it will be in A and at the 1st Class supplement price. If the flight is a transcon there will likely be 3 class flights with A ,D and M inventory. You should be able to buy a D class segment and then if you change to a 2 class flight get upgraded to A class. But if its not a transcon route with this option, I would think you could only buy in A or M at the First Class or Economy Class supplement respectively.

This is just my reasoned opinion no more!