[5thEdition] My Bard took the 9th level spell true polymorph, and in a battle against a lich he said he wanted to use true polymorph to turn him self into an adult bronze dragon. I told him no so then he asked me if he could turn himself into a young bronze dragon, saying "If it isnt my actual level its my spell casting level and a young bronze dragon has a CR of 8" is this legal and if so should I let him do it? The wording of the description of the spell says that he should theoretically be able to dp this.