#ahah & forms , how js function callback of #ahah can change #disable attribute

#ahah & forms , how js function callback of #ahah can change #disable attribute of other field of the form?

Let me explain better,

Working with forms, we define each field with the $form array

ex: $form['country'] = array (
...
'#disable' => true,
...
'ahah' => array ( 'event' => 'change', 'path' => 'change_other_field_disable_status_js' )
)

$form['zipcode' = array(
...
'#disable' => false,
...
)

after mapping the menu callback, to function 'change_other_field_disable_status_js' I tried everything I know(probably not much :D ) , in this function, to toggle the #disable from false to true of the 'zipcode' field....

The idea is to have dynamic rules for each field, regarding their input.

Please help, I'm really need a solution for this problem.