If you open the return then go and open the 1040X and put the year in the top of it, then go back and make the changes, both the original and corrected figures will flow to the 1040X automatically.

So if you already made the changes, prior to activating the 1040X...change them back to the original, then activate the 1040X and then make changes  OR just go and type the original figures into the 1040X manually.