Can I true polymorph into an Ancient Brass Dragon, and then use its Change Shape ability?

Question

If I were to cast True Polymorph (which permits all creatures, not just beasts, as the regular Polymorph does) on myself to turn myself into, say, an Ancient Brass Dragon (provided that I'm level 20, which is required due that dragon's CR) - could I then use the Change Shape "racial" feature of the dragon?

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...

True Polymorph:

Choose one creature [...] that you can see within range. You transform the creature into a different creature, [...]. The transformation lasts for the duration, or until the target drops to 0 hit points or dies. If you concentrate on this spell for the full duration, the transformation becomes permanent.
Shapechangers aren’t affected by this spell. [...].

Creature into Creature
[...] The target’s game statistics, including mental ability scores, are replaced by the statistics of the new form. It retains its alignment and personality. [...]

I can't find anything in the spell that would prevent one from doing so. The only thing that might come into play is the phrase "Shapechangers aren't affected by this spell". You as the caster are not a shapechanger, however, so even though you transform into a shapechanger, you aren't when you cast the spell.

Answer 1

Yes you can true polymorph into an ancient brass dragon

As you say, the spell and rules are clear. An ancient brass dragon is a creature, it is not a shapechanger (and may not matter if they were), and it is of the appropriate CR for your level.

Thus, there is no reason why you would not be able to use true polymorph in this way.

Ancient Brass Dragons are not shapechangers

Ancient Brass Dragons are not considered shapechangers. A creature is a shapechanger if they have the monster subtype (Shapechanger). Ancient Brass Dragon's creature type is dragon.

The fact that it is able to change shape (a special ability) is irrelevant to being able to transform into that shape. The dragon does not have the shapechanger subtype, thus it won't be potentially conflicted with the shapechanger subtype restriction.

You can use all the abilities of your True Polymorph form

The target’s game statistics, including mental ability scores, are replaced by the statistics of the new form.

Jeremy Crawford says:

When a game effect, like Wild Shape, says you use a creature's statistics, you use its stat block (see "Statistics," MM, 6).

According to that same section "Special Traits, Actions, and Reactions" are part of a monster's stat block.

Ability to change shape back to your old form is unclear.

It is worth noting that you may not be able to use change shape to change back into your old form: Can you use a Dragon's Change Shape Ability to Turn Back Into Yourself?