functions.php

function fb_change_search_url_rewrite() {
	if ( is_search() && ! empty( $_GET['s'] ) ) {
	  $keywords = get_query_var( 's' );
	  list($first_word) = explode(' ', trim($keywords));
		wp_redirect( home_url( $first_word ) . urlencode( $keywords ) );
		exit();
	}	
}
add_action( 'template_redirect', 'fb_change_search_url_rewrite' );