The older Metallic Dragons have the Change Shape ability:
Change Shape. The dragon magically polymorphs into a humanoid or beast that has a challenge rating no higher than its own, or back into its true form. It reverts to its true form if it dies. Any equipment it is wearing or carrying is absorbed or borne by the new form (the dragon's choice).
In a new form, the dragon retains its alignment, hit points, Hit Dice, ability to speak, proficiencies, Legendary Resistance, lair actions, and Intelligence, Wisdom, and Charisma scores, as well as this action. Its statistics and capabilities are otherwise replaced by those of the new form, except any class features or legendary actions of that form.
The dragon retains many things, but not explicitly its Challenge Rating (CR). What happens to a metallic dragon's CR when it uses Change Shape?
The CR remains the same (ex: CR 20 for Ancient Brass Dragon)
The CR is that of the assumed humanoid or beast (ex: CR 0 for Commoner)
I am honestly not quite sure; it could go either way.
The reason I'm asking is that in the module "Trust and Understanding", there is something that blocks creatures below a certain CR threshold from passing through. The effect reads:
Any creature that is of CR 16 (for characters, level 16) or less that enters this space must succeed on a DC 28 Wisdom saving throw or be subject to a permanent geas that delivers the command "Return the way you came."
If a Metallic Dragon that is normally over this threshold used Change Shape to polymorph into a beast/humanoid under the threshold tries to pass, what would happen then?