Merge walls of different thickness on same vertical without notice
You could taper the sections together with joint compound (though you wouldn't sand it into shape--that would be ludicrously laborious). However, you'd probably still see the transition at the floor and ceiling. 

Better, use furring strips on the new section of wall to result in a flush finished surface. 1/4" luan plywood or OSB is cheap and easy to work with, or rip your own from 2-by lumber.  

On a related note, I tend to prefer 5/8" gypsum board when I'm replacing plaster. It more closely matches the rigidity and strength of the plaster, and requires less furring. In some cases, none is needed.