Story / example: A Harpy gets Truly Polymorphed into a wyrmling Brass dragon. Over eight centuries she eventually becomes ancient and powerful – albeit a bit weird with her harpy-like personality and alignment.
As an avid reader of StackExchange she becomes a bit paranoid. She is concerned: should a Magical Dispelling get past her three daily uses of Legendary Resistance she would, once again, be that stupid, nasty, dirty little girl with those grimy-grubby wings… that happens to sing quite well. As such, she plots.
She sends herself into some Shifting Planes, ending up in Fell Shadows. Assuming success, she becomes one Shadow Dragon creature / template / archetype / true dragon / thingy.
"Aha!" she shrieks with delight, "Now i am genuinely a dragon… a DRAGON FOREVER!! Take THAT, Exchangers of Stack!" The cold hard rain falls as lightning cascades in the distance ominously. Indeed, has this mere harpy of a girl outwitted us?
Question: Does a transformation into an entirely different creature (such as changing types &/or transforming bodies) mean that Dispel Magic cannot undo a ‘permanent’ True Polymorph spell? Or does this apparently-totally-‘new’-creature take this polymorphic-dispelability along with them?