Undoing Heaven and Earth's transformation was theoretically possible. Noah would have to remove all the modified dark metal from the dragon, take away the organ that had learnt to produce it, purify the rest of the body, and shatter it so that it could regain its natural state.
The process would obviously feature issues that Noah had yet to decide how to handle. The modified dark metal was toxic for his existence due to its conflicting nature with Heaven and Earth's system. He didn't know how to approach it, but a solution would eventually arrive as long as he worked with King Elbas.
However, reverting the transformation would only bring the dragon back to square one, among the other magical beasts that were already losing against Heaven and Earth. That wouldn't help anyone. Noah would probably benefit from that act of defiance, but he wanted far more.
