Is it possible to reforge ancient Legendaries? And if - is there a 50% chance that it will go back to non ancient ?

From my understanding, you can reforge any legendary item and there is a chance for it to be ancient. So, if you reroll an ancient legendary, that chance is still the same so you may end up with a non ancient legendary.

Every reforge has 10% of an item to be ancient regardless of its previous stats.