Yoast/wordpress-seo — pull request #476 by haroldkyle (2013-12-19 04:29:55+00:00)
Title: Validated filtering of the changefreq values for posts (by post type), t...

...axonomies (by taxonomy name), and users.

See also:
http://wordpress.org/support/topic/plugin-wordpress-seo-by-yoast-need-to-change-sitemap-xml-frequency
http://wordpress.org/support/topic/sitemap-frequency-settings-daily-vs-weekly