The Changing Mirror

I had seen something about this a while back, but no followup. I'm starting a story about a young attractive professional woman turning into a nun, and I would appreciate any feedback the community could provide. Some of the aspects are:
What is the trigger for the change?
Is the change instant, or gradual?
How is the woman changed physically, mentally, emotionally, etc?
Is there magic involved?

Any and all suggestions are welcome.

It's not often I see changes like that; but I really do enjoy this kinda TF!

It really depends on the tone you want to take with the story and the narrative you want to develop, but there's a few ideas that come to mind for such a scenario.
First, in regards to the trigger - I feel the first step is getting her into a convent/temple/church/etc in the first place. One idea is that she gets wrapped up in a scandal and needs to lie low; or needs some sort of witness protection, and some convent in the middle of nowhere is where she's prescribed to stay; while having to adhere to the lifestyle of the nuns there. Alternatively - it could be a reporter who's doing a story on a secluded, monastic sect of nuns... Or, some sort of influencer doing a challenge? "I spent 100 days as a nun!? (gone wrong) (not clickbait)" :p

In any case, it'd be having to live like the nuns - with a particular emphasis on the demands of the mother superior, that could trigger things.

I'm a sucker for gradual changes, so I'm inclined to recommend that. Some... Taxing quality of what she's doing - and what's being done - to her causes her to display more changes every day. Touching on if magic's involved... It very well could be in this scenario. You've got a lot of room here. Everything that happens could be done practically - with chemicals, surgery, training or breaking her through either more explicit things or the sheer mundanity of her duties. Alchemy or more antiquated practices could be involved as a convenient way to get more extreme stuff done more quickly so the pacing of the TF is quicker - salves, poultices, herbal remedies; even incense could set the stage for why she changes in certain ways. It could also explicitly just be magic - presented as miracles or blessings from the mother superior. And regardless of what the actual cause is, the nuns could explain things away as miracles or punishments; either affirm that she belongs with them or to use them as ways to discourage her from certain acts.

As for how she changes, you've got a lot of room; again. I guess it depends on the aesthetic of the nuns... But, the broadest factor is modesty. Needing to conceal her body either because she's ashamed of what's become of it or because she's become so steeped in the doctrine of the nuns that she feels obliged to do so. As for specifics... Breast/asset reduction, developing a more pale, sallow, or freckled/mottled complexion, weight gain or weight loss, shaving/hair loss, her facial features could change to be more plain - or even ugly, with a rounder face or bigger or hawkish nose, maybe even a mole; and if she retains her hair, it could go from a vibrant or luscious shade/texture to something more plain and mousy. She could stay the same age; or she could even have some age progression thrown in to look the part of a stern older nun. Also, on that note, her demeanor could change - either being broken by the modesty she comes to embody and becoming meek, or becoming stern and stolid; all depending on the demeanor of the rest of the convent. Plus, depending on where the convent is, she could stand to pick up different cultural/racial qualities like an accent or race change.

Her mental state has a lot of room to change; as aforementioned, either losing her confidence or gaining some in being sorta brainwashed into a new sorta dogma. She could go from atheistic to a diehard for this kinked-up sect, her sexuality or views on it could change; depending on their dogma - and it'd be interesting if she sorta loses her knowledge of sex, flirting, romance, etc in the effort of becoming more modest. One thing I saw in another story, that I particularly liked the execution of, was using prayer and mantras as a means of brainwashing. Stuff like “A woman’s modesty is her virtue, and her virtue is her value.” - repeated a hundred times, by a dozen nuns on every side of her, every day... It'll get drilled into her eventually.

Hope ye like these suggestions; and I hope they help!