Why AJAX does not change field's value?
There are several problems with your code:


Your AJAX callback should return a form element. Now it returns plain 33.
All changes should be made inside form building function, in your case rah_cc_generate_form. Simply add condition to test $form_state['triggering_element'] and perform action if it was your AJAX trigger. Do not alter things in AJAX handler, it's not what it is for. I know it's counter-intuitive, but that's the way it is.
Changing default value once there is some user input will have no effect. Maybe you should try to change appropriate entry in $form_state['input']? But that depends on your intentions.