For a well-done one, what about Bette Davis in Now, Voyager? She is a very convincing frumpy and thick-ankled old maid in the beginning of the film but after a nervous breakdown, losing some weight and getting rid of her glasses, she emerges from the rest home as elfinly pretty as Bette could be.
If one could for one minute believe that Audrey Hepburn could be frumpy, I suppose one could buy Funny Face. At first, she’s supposed to be a dowdy bookstore clerk, but once glammed up by a beauty magazine crew, she turns out to be a supermodel. However, even if she’s a complete knockout as the supermodel, she’s beautiful as the clerk, too.
Well, I guess My Fair Lady basically repeats this same plot. Put a nice dress and some makeup on frumpy old Audrey Hepburn and hey presto - she’s a stunner. Duh.
Grease featured the most unusual turn on the “ugly duckling to swan” routine: Olivia Newton-John’s late in the third-act transformation from sugary-sweet virginal good girl to braless, spandex-tight-leather wearing, cigarette-smoking, trampy bad girl. Uh-huh.
Well, I suppose if you had already swallowed the premise that Jeff Conaway and Stockard Channing were teenagers in this film, that couldn’t be too much of a logical leap. But what a message for young girls - “If you want to win over the guy you like, you’ve got to look like a slutty biker girl!”
I didn’t buy her as an ugly duckling in that one but I thought she was pretty good at playing frumpy in Love Postion No.9, an earlier, really chessy comedy that seems to play constantly on cable…
Angelina Jolie in Gia. Not that they were trying to make her ugly in the beginning, but there was definitely a good and believable transformation.
Sandra did a far better (and more cliche nerd-to-beauty) transformation in (the forgettable) “Love Potion No. 9”
I also second Toula’s transformation in “My Big Fat Greek Wedding” as the most dramatic. I was stunned at how good she looked.
She’s ugly on the inside.
She looks much better in her transformation to every other character she’s played in every movie after that one.
Brittany Murphy had quite a transformation in Clueless (once again, not as big as her transformation into every character since Clueless).
They did a pretty good job of making Ally Sheedy look gross in the first half of Breakfast Club.
If I remember The Truth About Cats & Dogs right, it wasn’t that Janeane was supposed to be unattractive, just that she wasn’t supposed to be the stereotypical “blonde bombshell” kind of attractive.
I saw some movie where Julia Roberts was made to look like a gangly skeleton with horse teeth - Oh wait, that was after the transformation. 
Not used yet in one movie, but Cameron Diaz shows that she can be frumpy in Being John Malkovich.
Grease
For some reason I would not go out with Bad Sandy, mostly for fear os STDs but Good Sandy on the other hand I just want to get under that poofy skirt.
Blowero, that’s exactly what I kept trying to explain to my husband, before giving up in exasperation. It’s not that she is unattractive, it’s that she feels unattractive because she doesn’t fit the 5’11", 120#, legs-up-to-your-chin, tits-out-to-New-Jersey, blonde supermodel stereotype. Because she doesn’t fit that mold, she doesn’t think any guy could possibly find her physically attractive. Clearly, the photographer guy didn’t think she was unattractive, though; she’s the one he kept photographing while Uma was preening. The point of the movie was that she went through all these machinations, and they were all completely unnecessary, because he thought she was wonderful exactly as she was.
Rufus, they had Minnie Driver put on ten pounds or so to carry the implication that Benny was fat. It’s all much clearer in the book; Benny’s not fat, exactly, but she’s big and solid-built, and she feels fat and thus unattractive.
That one always bothered me as well. At least in the play Danny changed himself to appeal to her - a much bigger deal is made of the fact that he turned into a preppy jock “Letterman” to please her. Makes it a little less offensive and a little more Gift of the Magi-ish.
While she’s never ment to be ugly, Catherine McCormick in Dangerous Beauty does a wonderful transformation from merely pretty girl to stunningly gorgeous courtesan.
Not quite the taking-off-her-glasses type transformation, but in Ruthless People, Bette Middler is kidnapped by a particularly clueless couple. During her month-long or so incarceration in their basement, she starts lifting weights and doing sit-ups and before you know it, she’s lost TWENTY POUNDS and is ecstatic. 
She was played by future Oscar-winner Dorothy Malone
Another good example is Lili Taylor in Dogfight.
They had this plot on Gilligan and Brady Bunch. Curse you, Sherwood Schwartz, for your lack of originality!
True dat.
Oh, now that would have have made so much more sense. I remember leaving the theater after seeing the movie just scratching my head, thinking the moral of the story is - be a slut?" Why on Earth did they change the premise?
In America’s Sweethearts, Julia Roberts was supposed to be playing the overweight sister (and assistant) to Catherine Zeta-Jones’ character.
Kathleen Turner did a fair transformation in Romancing the Stone. When we first see her, she crying – red-faced and slobbering – over the happy ending she wrote to her romance. After a series of adventures in South America, there’s a particular scene where she’s bending over, picking flowers in a field, in a slit skirt and peasant blouse.
zing!
I always hated Grease for exactly the same reason. IIRC there was a token nod to the ending of the stage version, in that Danny does briefly appear in a letter jacket and seems to be making some effort to change his image…but he tears off the jacket and reverts to his old self as soon as he sees the new Sandy. Ugh. I guess the moral is, “Guys, you may think you have to change in order to impress a girl, but actually you don’t. Just be yourself. Girls, on the other hand, should completely alter their appearance and behavior in order to comform to male desires.”
Not actually on topic for this thread, but I have to give a shout out to Michelle Meyrink as Jordan Cochran in Real Genius. She’s the nerdy, “unpopular” girl with bad hair who dresses funny. She’s totally hot, of course, but her personality is so hyper-active and generally wierd that you absolutely buy the character. And the best part? The main character falls in love with her because she’s wierd and hyper-active, and at the end of the movie, she’s still wierd and hyper-active, and she still dresses funny and has bad hair, because that’s what the main character fell in love with!
Dammit, where can I find a sexy super-genius physics major who can knit me a sweater during a study break?