but i dont really think that old emrys is merlins true form

i think he stepped into the light

for one moment - everything was clear, everything was pure, full of magic, full of merlin himself, because he is magic

then he knew what he had to do

and i think he changed into old emrys, because old emrys is the epitome of “PLEASE GOD NO” for morgana since forever, so when morgana sees merlin again, its not only “oh, you survived”, but “oh shit you have your magic” and i think that his appearance as old merlin is just his next step and is a hint for us whats about to come in a few days