Calling ajax function from jquery on change event
Can I attach an ajax call to an on change event that will reload the
  text field without reloading the page?


If that's your question, then the answer is yes, you can. 

If you meant to ask how to do that, then please refer to Ajax Forms documentation for Drupal 8 or Drupal 7.

If you hit any problem when implementing it, then come back giving us the details of specific issue as provided information does not describe what the problem is.