[jQuery] validate - How to dynamically set required:true to required:false

[Ken]

I am dynamically changing the className from required:true to
required:false but jquery.validate.js always is displaying that this
field is required.

function change_required(id,bRequired) {
document.getElementById(id).className=
(bRequired?'{required:true}':'{required:false}')
}

Before submit I see that input fields whose className is dynamically
set to {required:false} is still be treated by jquery.validate.js as
if  {required:true}

This is the call to validate for the form:
jQuery(document).ready(function() {
    jQuery("#form_id").validate(
    {
      errorClass: "form_error",
      errorElement: "div",
      wrapper: "span"
    });
})

How do I set a rule to ONLY validate when the className has
required:true?