The makeover sequence is pretty much a staple of Disney's animated movies. How will you sell more toys if the princesses (and princes) don't wear multiple costumes? But beyond that callous, calculated money-grabbing angle, there are plenty of Disney movies that have 100 percent, totally legit story-based motivations for their princesses costume changes:
After chasing her sister Elsa through the snowy wasteland outside Arendelle, the summery-dressed Anna realizes she is very, very cold. So she heads to Wandering Oaken's Trading Post and suits up in an outfit far more conducive to winter. It's a little change, and probably not important other than in the alternate universe where Anna freezes to death halfway through the second act of "Frozen," but it is legit. Totes, totes legit.
The long-tressed princess is sobbing over her beau Flynn Rider, who has been fatally stabbed by Rapunzel's adopted/witch mother, when, rather than letting the evil Mother Gothel continue to abuse Rapunzel's healing powers he cuts her blonde hair off... And it turns into a brown bob! She may have lost her healing powers, but at least she got a fabulous new hair style!
Tiana, "The Princess and the Frog"
So many legit transformations in this movie. Tiana turns into a frog, of course, which is the whole flippin' plot; but she also gets dressed up to go to a fancy party, where she's mistaken for a princess by the similarly transformed Prince Naveen. If she hadn't gotten spruced up by her best friend Charlotte, nothing that follows in the movie would have occurred. Also, there's the dude that gets turned into Prince Naveen, but we won't get into that.
They make a man out of her. That's the whole plot of the movie. Legit.
Belle, "Beauty and the Beast"
Disney's most voracious reader has numerous costume changes throughout the Oscar-nominated movie, but the only one that actually pushes the story forward (sorry, Snowsuit Belle) is the much-lauded dance sequence at the film's center. On the surface, sticking Belle in a pretty yellow dress isn't totally necessary; but when you realize that it's more about matching the finally dressed up and less monstrous Beast, versus showing off how Belle is "finally beautiful," it makes a lot more sense. The less said about the Beast's transformation at the end though, the better. Yech.
Ariel, "The Little Mermaid"
It's hard to pick one transformation Ariel goes through in particular. There's the completely legit story-based transformation that finds the little mermaid growing legs and finding out where the people go. There's her wedding dress. There's that time she got in a frilly pink outfit and brushed her hair with a dinglehopper. But our favorite is the sail dress she wears to first seduce Prince Eric, because come on. That's awesome.
The grandmama of all Disney princess transformations, Cinderella goes from rags to riches (and back to rags) during one magical night. It's the plot that caused every other plot on this list, and stands the test of time as one of the greatest makeover stories ever told... Other than "She's All That," of course.
Bonus: Elsa, "Frozen"
Technically she's a queen by this point, not a princess, and doesn't count. And we can't argue that her fabulous makeover is more character motivated than plot motivated -- would things have changed that much if she hadn't donned a sparkly blue dress? -- but damn, girl. Get it.