How can I override one post and make it display content for another post?

pre_get_posts hook should work for this:

add_action( 'pre_get_posts', 'wpse35832_pre_get_posts' );   
function wpse35832_pre_get_posts( $wp ){
    if( $wp->query_vars['p'] == 200 ):
         $wp->query_vars['p'] = 100;
    endif;
    return $wp;
}

It may work a bit differently if pretty permalinks are used, different query vars may be set, not sure though.