Arrays: Cómo crear un array cuyos índices son los valores de otro array

<?php
     $gran_array1 = array("1940","1941","1942","1943","1944","1945");
     $gran_array2 = array_count_values($gran_array1);
     print_r($gran_array2);  // da Array ( [1940] => 1 [1941] => 1 [1942] => 1...
?>