It give a hook to init your function for the requirements. The hook {status}_{post_type} is your right hook. If this hook was fired, you can start your custom function to change the post type. For your custom post type “prototyp” use publish_prototyp.
See also the codex about the hook, also the hook from old status to new status http://codex.wordpress.org/Post_Status_Transitions
For the changing of the post type is the function set_post_type your right choice. See also the codex page for more information.
http://codex.wordpress.org/Function_Reference/set_post_type