Another great headcanon to work with, and one that makes me want to see more non-contemporary changeling stories take place.

While changeling chitin armor would be effective, it would not be nearly as resilient, nor as flexible, as the armor worn by ponies traditionally namely that of steel and gilded metals.  Changeling hide, within Papi canon, is harder than pony hide, but it is still soft and pliable in comparison to steel.  It is kept soft by the changeling’s body, its fluids and its heat.  After death, a changeling’s hide will harden and become very stiff.  At this point it does work well for strength, but not for motion as it does not flex, and it is not as easy to mesh pieces of the flesh together into plate or mail as it is with metal.

Now, depending on the canon of someone’s story changelings can predate metallurgy in Equestria very easily, they are insect like after all, and this hints at a long history.  Within Papi’s canon however, they are relatively recent being around only slightly longer than the Alicorns which came about shortly after the end of the wars of the three pony tribes.  As such, metallurgy was already well established and a fairly prominent aspect of both Unicorn and Pegasus society, making changeling armor completely inadequate for either of the races.  This of course leaves Earth ponies.

Earth ponies would arguably benefit the most from changeling armor, being naturally stronger and more resilient than their winged and horned counterparts.  Still, it would be easier for the race to trade foodstuffs for metal than it would be to fashion usable armor out of changeling hide.