Transformations I like are slow, often self-inflicted, involve reduced status and happen due to stupidity or bad luck rather that someone's evil intent per se. Although, of course, there is nothing wrong with an occasional evil character destroying other people's lives. The stories on this list range from relatively mild and mostly psychological to nasty and brutal and most feature non-consensual stuff so please be warned. In no particular order:
Career Opportunities - and up-and-coming female executive is caught in the web of office politics (largely of her own making) that gradually turns her into a penniless and sexually frustrated bimbo secretary to her former equals. This early 1990s classic from Parker is one of my all-time favorite stories. Its slow pace, a well-developed plot and realistic motivations of main characters set it apart from countless of other boss-to-secretary or exec2sec stories. With not a hint of mind control or magic, the plot is entirely driven by the main character's own stupid actions. A true gem.
A Certain Perception - Angel Cherysse wrote what could easily be one of the most detailed (and frighteningly believable) transformation stories out there. It's a tale of unparalled obsession and brutal psychological self-destruction like no other. I am running out of superlatives here, but if you haven't read it, do yourself a favor. If you are not into TG fiction, let this no stop you. Yes, it's a story about a man who willingly (and very, very slowly) transforms himself into a female hooker, but more than anything it's a story of social and cultural drop done with painstaking attention to detail.
High Cost of Tanning - racial transformation is another favorite theme of mine and one that is quite rare compared to some other transformation types such as age regression or sex change. A female's racial transformation (and a relatively realistic one at that) is even rarer. Julien Sorel has penned a number of brutal transformation stories, often involving turning women into inanimate objects such as plants or human cows. High Cost of Tanning is quite different though. It is about a white woman, who, due to a mistake at a tanning salon, becomes far darker than she wanted to be and is mistaken for a native girl during her vacation. It's a gone native story gone very wrong.
Substitution Games - a successful businesswoman must impersonate an escort to avoid losing an important client, which leads to her gradually becoming an escort once and for all. This story by Anton Psychopoulos combines several elements that I love in transformation fiction: "stuck as someone you impersonate" trope, social/cultural drop, becoming a prostitute, stolen identity. I only wish it were longer. Highly recommended.
Flower Girl - There is a huge adult baby/diapers community out there that I was never really a part of. I do like age regression stories, but my favorite change is adult-to-teen (or 10-11 years old at the youngest). I like themes of loss of power and authority, but not when it becomes absolute and irreversible (like with babies) and diapers don't do anything for me. However, I can highly recommend this excellent story by Daring Diane for its perfect portrayal of gradual loss of status - from a young adult to a little girl, from a bridesmaid to a flower girl.
Out of the Frying Pan, Into the Fire - another stuck as someone you impersonate/game gone wrong scenario. A young female executive pretends she's a lesbian lover of her boss as a way to get rid of her bothersome boyfriend. Obviously, not everything is as it seems at first and the game slowly but surely becomes the heroine's new reaility. A (heavy) BDSM epic by Musker.
Lady Jane Greystone's Remarkable Experiment - another Parker classic. A Victorian lady scientist's discovery of an African primitive tribe of half-apes threatens to unravel all the notions of creationism. Before she can make her discover known, however, she is forever stuck as one of the savages. An extremely erotic and graphic "gone native" story with very strong social drop/downgrade undertones.
Bangkok Slaver Series - began by Parker and penned by several authors after that, this long series is a joy to read. It details misunventures of several Western women, who are forever stuck in the sex underworld of South-East Asia. The only thing missing for me is cultural change (although there are some minor hints), but you can't have it all, can you?
Allan Aldiss wrote incredibly well-researched and desriptive North African harem erotica, much of it fictionalized, of course. Many stories are set in early 19th century or Victorian period and deal with English ladies stuck as sex slaves to Middle Eastern men and women and forced to commit unspeakable things. Very high quality writing, which is, unfortunately, rare for the genre. This is BDSM erotica at its very best. Some of Mr.Aldiss's works include Barbary Slavedriver, Barbary Pasha, and White Slaves for the Sheik .
Ghetto Nightmare - the old story by Glaucus was the first "roleplay gone wrong" piece I've read that got me interested in the genre back in the day. That gives it an honorary mention. A husband and wife's prositute-and-client sex game backfires spectacularly with the wife stuck as real hooker to the black gang. It also has my favorite theme of cultural change with the wife slowly adopting speech and manners of the ghetto inhabitants, talking and behaving like a barely literate black hooker. For me, that is far sexier than actual sex scenes. I am strange this way.
Almost There Already - a senator's daughter pretends that she's a lesbian to avoid being punished at school, but then things get completely out of hand and she's forced to continue pretending until there is no way to go back. I really like stories by Couture and this one is no exception. What sets this one apart for me is the theme of "stuck as someone you impersonate" as the character slowly digs own grave deeper and deeper.
Transformation of Betty - a 16-part opus magnum by dom_u_softly is another favorite. Turning a shy wife into a sex monster is hardly a new theme, but here it is done with such vivid detail and with such attention to step-by-step psychological change that it's impossible not to recommend this story to lovers of downgrade fiction. A husband gradually turns his wife into a slut for other men and women.