verbs - switch genders or gender?

Recently I read an article on Wired.com "Apple Hires Hacker Who Helped Save Windows From Security Hell", http://www.wired.com/wiredenterprise/2012/12/apple-hires-hacker/

Here is a sentence I'm kind of confused:

Back then, Paget was known as Chris. She switched genders last year.

Why it is switch "genders" instead of "gender"? Thanks