Arrays: Pasar los índices de un array al contenido de un nuevo array

<?php
$gran_array1 = array("1940"=>"James Stewart","1937"=>"Spencer Tracy","1951"=>"Humphrey Bogart");
$gran_array2 = array_keys($gran_array1);
print_r($gran_array2);   // da Array ( [0] => 1940 [1] => 1937 [2] => 1951 )
?>