It started small. Undergarments taken away for washing were replaced with slightly frillier, slightly less covering undergarments. Strange, but perhaps this was simply a new style. Nobody would see them anyway.
Meals began to be served personally by the head maid, rather than the pair that it was typically assigned to. Occasionally, there was a slight flavor that was ever so off, but the chefs were adamant that the supply of ingredients was the best available, and the head maid even offered to personally test the meals upon delivery. It brought peace of mind, but the taste remained. Eventually, it was easy to get used to it.
The bigger changes were when the young royal started to feel concern. Every morning, the silhouette in the mirror was ever so slightly different, but the changes were adding up. Clearer skin, rounder hips, a chest that looked... fuller?
Many of the maids had started to take notice, though few said anything. But more began to change. Hair no longer styled as it once was. Undergarments that were almost undeniably a woman's. The latter was welcomed, begrudgingly - the head maid pointed out the necessity, now that the young lord's chest was starting to be noticeable through clothing. Occasionally, "my lady" was spoken in error, before being hurriedly corrected with profuse apology.
A maid dress was mixed into the washed garments, one day. At first, the young royal considered calling for the maids. But standing in the mirror, noticing the figure that reflected back, more like a young lady than a prince, curiosity won out.
It fit perfectly, as though it had been tailor made for precisely this purpose. Watching with fascination as the maid in the mirror moved to the lord's command, a hand slipped under the maid's petticoat, and the prince watched with fascination as this woman exposed herself, touched herself for an unseen audience, the image of a perfect maiden except for a reminder between her legs that it was not, in fact, just another of the servants of the manor.
The head maid chose this moment to, for the first time in her career, enter unannounced. Silence filled the room, the absence of girlish moans suddenly processed by the young lord, nowhere to be seen in the mirror.
The door was shut before anything could be said. The maid dress was removed hastily, tossed into a far corner of the room. An indignant speech was prepared for when next the maid was seen, warning her that her career, if not her life itself, was forfeit, were she to speak of this to anyone.
The following morning, the head maid arrived to dress the young royal, and the prepared speech evaporated. A new set of undergarments, more brazenly effeminate than the last, were slid on with less protest than there should have been, and a noblewoman's dress followed. The head maid gathered a number of clothes and left without a word, and the prince did not leave their bedroom that day, nor did another maid ever enter, save for the head maid.
When, in the coming morning, the entirety of the wardrobe had been cleared out and replaced, there was again no protest, and the noble was dressed again in silence. When the head maid wished a good morning to "my lady" on the way out, the princess did not object.