Bevendre

Depending on the canon one works with with changeling social structures, conversion means and interspecies relations, this is very doable.  It doesn’t fit with Papi’s canon for a few reasons, namely that conversion can only be done by a Queen and through immediate proximity (In a more violent hive, should a pony be taken forcibly and converted, only to have the process fail then the pony would be eliminated, or possibly eaten), and that changeling hives are hidden by design.

Changelings, in Papi’s canon, are extremely loathe to show themselves because of a long history of prejudice, and attempted genocide of their race by ponies who see them as monsters and demons.  In this regard, a hive invasion force would be a fitting construct, but first the hive would need to be discovered at which point it would be easier and faster to completely destroy the structure than it would to infiltrate it.

Being immune to conversion, a process that overrides the physiological aspects of the being in question and alters them to a changeling form, though possibly through a random genetic anomaly that resists mutation of the organism, would most likely be a hereditary product stemming from being the offspring or direct relation to a converted drone.  However, this could work the other way, making the individual more susceptible to conversion, assuming that they are whole and functional enough to reach that point.

It’s a good idea, and one that works well in many headcanons and plots, but doesn’t sit well within Papi’s canon.