This is based on my observation that Chrysalis does not resemble her children/subjects and the black form is the one they're born with.

Stage One-Larval changelings. This is their first form. Wrapped in a protective sack to protect them from predators, small, still no pupils.

Stage Two-They grow into their black form with a holed carapace. They feed off love and are aggressive. In an ideal hive environment, they are fed by adult changelings, but this state can last indefinitely.

Stage Three-Sometimes referred to by the offensive term "skittlebug", this is one of their adult forms. In this state they are colorful and highly social. This state is achieved when they develop enough emotional maturity to share love. Their bright colors and gregarious personalities may be for attracting mates.

Stage Four-This stage might only be achieved by certain females. It is the Queen form. The adult colors fade to black and they grow a mane and tail along with pupils. (How does this effect their visual processing?) They may become aggressive in order to find love with which to feed their offspring. (Or Chrysalis was an atypical, unusually aggressive queen.)

I believe that their black holey forms was just there defalt because that's how their queen looked.

My headcanon is all common drones are given set parameters for their appearance based on their King or Queen's preference, while Elite changelings(Like Pharynx) are given more leeway with how they may appear.

When Thorax became king, all the changelings changed to reflect their new leader, and since he is much more understanding than Chrissy, they were all given a lot of leeway.

7174652
So they can look like anything with no "default?" Cool.

I think all (unreformed) changelings look like the regular changelings. Yes, that is all of them, including Chrysalis. They are shapeshifters, after all, their appearance can be anything they want. I believe most ordinary changelings, being somewhat love-starved, don't wish to waste energy keeping up a facade for no effective purpose. I am not of the belief that they are mindless drones, but none of them seemed to care much about trivial things like appearances. Chrysalis, however, immediately proved herself to be vain and narcissistic, and it would make absolute perfect sense for her to want to have the appearance of a being much more powerful than an ordinary-looking changeling. I'll note that ordinary changelings were originally kind of small, appearing just a bit smaller than ordinary female ponies. Also remember that she just so happens to be about the same height as Celestia. See I don't think that's a coincidence, I think it is by design. She sees herself as this conquering queen, so she models her physical appearance on the only available correlating counterpart: Celestia. Also of note is that Chrysalis is sort of the evil dictator of a society of people facing mass starvation, and it fits her M.O. that she would waste precious energy to maintain her appearance as a show of power and strength, in addition to her own vanity.

7174652 I will say that I like the idea that the skittlebug look is derivative of how Thorax looked. But um... didn't they all transform at the same time?

7174662
A lot of people think they have some kind of hive mind...

and even if they don't Thorax transformed a full minute before everyone else did.

7174650
My changeling headcanon tends to fit the world of the story though I keep most of the MLP canon.
The ones with bug like eyes are drones, and the other ones are royalty.
They feed on emotions besides love, and some even feed on hate.
They have a weak kind of hive mind which makes them great collaborators, and work well in groups.

In one story, a bunch of changelings lived in Manhattan with other creatures and are seen just as regular residents.

In another story, they tried to steal emotions from humans in another universe, but instead triggered an invasion by alien creatures. Then they had to ally with a resentful Celestia. This story had some changeling politics and some insights into their culture.
And I added a clan naming scheme.

Clan name, personal name.
Chr'Ysalis

Unlike the Hall of the Virgin Queen’s First Flight or even the Hall of the Dutiful Drone, this room was dark and somber. There were no pennants or banners. It took Chr’Aya a moment before she realized why. The stink of fear was a good clue. Since this was a room for those that awaited judgement, some changelings didn’t wait. To them it made sense for them to kill themselves. So the room was empty of anything that could hasten a changeling to the Final Hive. That thought made a chill race down her back. Chr’Aya didn’t like to think of the stories about what happened to changelings when they died. The stories that her nurse told her about the UnderHive crawled back into her mind’s eye.

Youngling Chr’Aya looked up at her nurse. “Tell me about the UnderHive again, please Nursey.”

Nursey grimaced. “You shouldn’t be so curious about the UnderHive. There is a place for those that look for knowledge beyond what the Hive teaches. Fanged ch’kals with snapping jaws roam the dark halls for blind changelings with milky white eyes. When you are judged by Bykriss the Just and sent there your eyes turn white. Then you’re stumbling and fumbling through the darkness until a ch’kal finds you. Little one, you don’t want to know what it does to you then.”

Youngling Chr’Aya, just shrugged. “What’s a suicide?”

Nursey just sucked her fangs. “Tchi tchi, little one. So many questions. Suicides are changelings who kill themselves. They go straight to the UnderHive without judgement. There they go to the Floor of Flailing so razor taloned Ekreet can walk on them forever. Can you recite the hells for those who hurt and betray the hive?”

Youngling Chr’Aya stood up on her hind hooves and closed her eyes. “The black sun of madness, the rain of malice, bugbringer winds, the rot of flowers...” When youngling Chr’Aya was done, Nursey gave her a bit of larval honey.

In another story that was not published yet, I wrote about changelings who were living on Earth. Of course, no one knew that they were changelings.

7174650
the only current history we have for changelings comes care of chrysy and a certain comic that hasn't completely been refuted by the show as of yet and likely never will be, weather she was telling the truth or not we don't know

but basically, chrysalis was the first changeling, all others descend from her and she was born from a plant that was contaminated by wild magic (a form of venus flytrap specifically)

sooo basically even chrysalis didn't know about the 3rd possible state till it happened
its theorized that chrysalis own 'skittle-bug' form wouldn't look very different from her current one just sans the holes and with a different wing type (like her 'good' counterpart in the reversed world, with the butterfly wings)

7174667
his brother however changed AFTER everyone else and atained the 'stag' form too, though a smaller one...i think harmony may have played a part in that, she seems to like ruleing pairs...