Changeling physiology miraculously shares traits both of an insect and mammal. While having almost similar to ponies internal structure they do have an insect-like outer shell that they change with age and time. For usual changelings this is routine, but this concept doesn't apply to the queen. Unlike them, her shell grows considerably slower and stays skin-like for a much longer period of time, due to their naturally defined place inside of circle of loyal guards. Therefore queen tries to maintain her inevitably damaged exoskeleton as long as she can untill she is in complete and guaranteed safety or is defeated.
Ironically Chrysalis were to change it because of both. Queen has to be ambitious, self sufficient, and should never take a step back, to be an exemplar for her ponies (or changelings if you prefer it that way), to be a beacon of certainty in the mist of future. Therefore she had to refuse an offer to change and share love not because she meant it, but because it was dictated by rules which she had to follow for aeons. Alone, torn, confused, eventually she had to embrace new reality and "give herself up". Poor thing plodded into crystal empire, as the only place she knew, waiting for trial and imprisonment. Such a thing wasn't even in the mind of hosts, even after her swear of vengence, but something had to be done to ensure that queen (only in biological meaning now) changes once and for all. Admittedly, it was done more from care for her health than from playing along, but Chrysalis' "punishment" was to get rid of her ancient shell in front of Celestia and Luna, Twilight and Cadance as her "judges".
After that came the strange time of normal life. Poor Chryssi now has to smear strange minty compound on her teeth with little brush twice a day, wear strange cloth pieces even if she's not cold, eat bitter tablets from… something named as weirdly as anything else in ponies world (she actually thinks they make her eat them as part of her punishment), and most importantly she now has to go in "spah". Nothing, even calcium pills, was such a problem for Twilight as spa was. You try explaining Celestia-knows-how-old changeling queen, who knew nothing but a tyrant life inherited from her predecessor, why those ponies touch you in all manner of places, and more importantly… why it feels embarassingly good?