Question

suppose that the accepted anatomical position is to have the palm facing backward. and how would this new position change your descrition of the palm and thumps

Answer 1

If the anatomical position changed, that means the name of the scientific name for that part is changed too.

In medical term, the word that used to indicate a location determined by the recognized anatomical position. Dorsal(back) and ventral(front/belly) is one of those words. 
In this case, the palm anatomical position is changed to palm facing backward. Then the way to refer backward of the palm changed from dorsal into ventral.