You lost the big picture joker, none
Reform changeling are a treat and steal love energy from the pony and go kidnap a pony and take thier place and feed on the love of the friend and family of the pony he or she have take. And for those believe changeling transform pony in their kind are wrong. No the changeling use them victim as love stock to feed later or use them put thier egg in female pony for keep the eggs wham for hatch them and do this again, male pony are use for love stock or brainwatch them and use them to make thier cover believable, the idea of then transform the pony in thier is dark sure but is not possible plus if the changeling do this to all thier victim the’re will no prey left to gain love with plus is will be see as inpurity, sure there is some good story pony become changeling, like the one who twilight become a changeling queen but there is more hive and queen but chrysalis be see rogue queen and go against the high queen and see pony as nothing but prey and see a pony become a changeling and queen as blasphame, well better see read the story on fimfiction.com

Because the pony disguised as a changeling is just a pony in a costume and not a real threat to Equestria.