How to modify the existing field in a form?
You'll usually find the elements for fields buried in the form and you'd alter them like so:

$form['field_my_field']['und'][0]['value']['#ajax'] = array(
  // ajax settings
);


I strongly recommend installing the Devel module and using the dpm() function to perform debugging. You can use it to print any variable to the standard message area, and it allows you to inspect arrays and objects.

You could easily find out where abouts in the $form array the fields you're looking for are by including a line like this in your form alter function:

dpm($form);