are there any animie that is about sex change?

Kampfer

Ranma 1/2 - Main character is Ranma Saotome, heir to the Anything Goes Style of Martial arts. His father took him to cursed springs of Junsenkyo, where Genma Saotome falls into Spring of Drowned Panda. Ranma fell into Spring of Drowned Girl, and thus, with cold water, Ranma becomes a red haired girl when splashed with cold water. Wackiness ensues as Ranma (girl type) ends up meeting the Tendo family; Soun Tendo, a old family friend of Genma's, and his daughters, Kasumi, Nabiki, and Akane. Genma has a habit of marrying his son off for food, or other things, so there are a lot of girls who are engaged to be married to boy-type Ranma: Shampoo, the Chinese Amazon, Akane (tomboy), Uukyo (the Okiyamiaki chef). When Ranma's mother Nodaka comes by for a visit, Ranma assumes his girl form and poses as Akane's cousin, Ranko.

The one I am the most familiar with is Ranma 1/2. I think I have only seen one episode of Kampfer so I can't say much about that. Also Ranma 1/2 has been broken down enough in what Kristin Rothacher said.

Ranma 1/2 is a classic show based around genderbending.

If you want something more recent, I've heard of some shows such as Kampfer and Birdy the Mighty Decode that do some similar stuff.

Bear in mind I haven't actually watched any of the above and can't give any guarantees regarding them.

The most famous anime series about sex change has to be Ranma 1/2. Two other people who answered before me have given you a sufficient description, so I won't add to them, other than to point out the typo in the latter one, from Leah - "Not" at the beginning of the one sentence should be "Now".

Another (shorter) anime series that is about sex change is Kashimashi, subtitled Girl Meets Girl. I haven't watched it yet, but I've seen summaries of it on the World Wide Web, and I'm going to buy it as soon as I get enough spare money to do so. It's about this effeminate boy named Hazumu who gets hit by a UFO and killed. When the aliens on this spaceship reconstruct him and bring him back to life, they make him a girl. The comedy series chronicles his adjusting to living life as a girl instead of a boy.

OURAN HIGH SCHOOL HOST CLUB - really funny, totally lovable characters and romance. Dubbed 26 episodes.
a girl starts at a high class school, where only the rich children go. She breaks a vase from the host club and has to work her debt off. She hides the fact that’s she a girl from the rest of the school and dresses like a boy. Because the host club is all boys and after awhile they all become good friends, especially with the king of the host club.

RANMA 1/2 - action, comedy, martial arts, romance, dubbed, just under 200 episodes. by the same author of inuyasha. Ranma was training in china with his father when he fell into the cursed spring. Not when he is splashed with cold water he turns into a girl and his dad turns into a panda. They move in with Akane’s family because both of their parents have decided they want them to marry and take over the dojo.

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I recommend Wandering Son:
http://www.youtube.com/watch?v=_HMqrs6SvV8&feature=related