If we summon a Couatl with Conjure Celestial, and the couatl uses its Change Shape ability to turn into a lower CR humanoid - say, a Hobgoblin Devastator with various level 1-4 wizard spells - does the Couatl get those new spells in addition to (or temporarily replacing) its existing spells?
Change Shape: The couatl magically polymorphs into a humanoid or beast that has a challenge rating equal to or less than its own, or back into its true form. [...]
In a new form, the couatl retains its game Statistics and ability to speak, but its AC, Movement modes, Strength, Dexterity, and other actions are replaced by those of the new form, and it gains any Statistics and capabilities (except Class Features, legendary actions, and lair actions) that the new form has but that it lacks.
Based on this, it might be getting the spells.
Reasons it might not be getting the spells:
- the Couatl has Innate Spellcasting and the hobgoblin devastator has Spellcasting, maybe it "already" has spellcasting so it doesn't get it, since it "gains any ... capabilities ... that the new form has but it lacks"
- the 9th level Shapechange spell stipulates that you don't get spellcasting abilities of what you change into, and "Shapechange" is similar to the spelling of "Change Shape" and might have the same intention
- the Spellcasting ability might be considered a Class feature, although the Hobgoblin Devastator description does not say it is a particular class
I did some searching and didn't find anyone asking this particular question. Is there a ruling somewhere, or something definitive in the rules that I'm overlooking?