$arr = array(
    array("id" => 1, "something" => "etc"),
    array("id" => 2, "something" => "etc"),
    array("id" => 3, "something" => "etc"),
    array("id" => 4, "something" => "etc"),
    array("id" => 5, "something" => "etc"),
    ...
    array("id" => $over9000, "something" => "etc")
);

$newarr = array(
    array("id" => 1, "different" => "etc"),
    array("id" => 2, "different" => "etc"),
    array("id" => 3, "different" => "etc"),
    array("id" => 4, "different" => "etc"),
    array("id" => 5, "different" => "etc"),
    ...
    array("id" => $over9000, "different" => "etc")
);

$newarr = array_map(function($i){
    return array(
        "id" => $i["id"],
        "different" => $i["something"]
    );
}, $arr);