The Deva stat block states:
Change Shape. The deva 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. 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 deva's choice).
In a new form, the deva retains its game statistics and ability to speak, but its AC, movement modes, Strength, Dexterity, and special senses 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.
According to the above, I believe if a deva used change shape to magically polymorph into a cat, it would have:
- Armor Class 12
- Speed 40 ft., climb 30 ft.
- STR 3 (-4)
- DEX 15 (+2)
- Lose its Darkvision
- Gain Keen Smell and Claws attack
However, would its size remain Medium or change to Tiny? If it maintained its Medium size, how would this effect the creature's appearance and game mechanics?
Also, let me know if I've missed anything regarding the stat changes.