Any combination is possible, even custom post types:

• Page to Post
• Post to Page
• Page to Attachment
• Post to Custom

Note: Invisible post types (revisions, menus, etc...) are purposely excluded. Filter 'pts_post_type_filter' to adjust the boundaries.

Now with bulk editing, thanks to Matthew Gerring!

• Install the plugin into the plugins/post-type-swticher directory, and activate.
• From the post edit screen, above the "Publish" button is the "Post Type" interface.
• Change post types as needed.

Version 1.2

• Add bulk editing to supported post types
• Props Matthew Gerring for bulk edit contribution

Version 1.1.1

• Add is_admin() check to prevent theme-side interference
• Change save_post priority to 999 to avoid plugin compatibility issues
• Remove ending closing php tag
• HTML and PHPDoc improvements

Version 1.1

• Fix revisions being nooped
• Fix malformed HTML for some user roles
• Classificationate

Version 1.0

• Fix JS bugs
• Audit post save bail conditions
• Tweak UI for WordPress 3.3

Version 0.3

• Use the API to change the post type, fixing a conflict with persistent object caches
• No longer requires JavaScript

Version 0.2

• Disallow post types that are not public and do not have a visible UI

Version 0.1

• Release

Why would I need this?

You need to selectively change a posts type from one to another.

Does this ruin my taxonomy associations?

It should not. This plugin only changes the 'post_type' property of a post.