@JeremyECrawford went back and re-listened to the polymorph Sage Advice and I have a follow up question: once a true polymorph spell is made permanent from concentrating for the full duration, does reducing that creature to 0hp still cause them to revert to their original form?
— Eugenio Vargas (@eugenio_vargas) January 2, 2018
Is one way to read it. Another, and one that I found a tweet from @mikemearls backing, is that once it’s permanent, it’s PERMANENT and magical intervention is required even if the creature is reduced to 0hp.
— Eugenio Vargas (@eugenio_vargas) January 3, 2018
The text of the spell says it has no effect on a creature with 0 hit points. That statement is made after the bit about lasting until dispelled. At 0 hit points? The transformation ends.
— Jeremy Crawford (@JeremyECrawford) January 3, 2018