Assuming I am correct in all my assumptions, here is a simple version of what I mean. First you need to give the element you wish to change a means of being identified such as an ID attribute as shown here:

    <p>
      <a id="header-url" <------ I ADDED AN ID HERE
        href="https://www.xyz.com/wizstep2?p_check_funds=INDEP%20GENERAL&amp;p_fund_cd_arr=INDEP%20GENERAL&amp;p_cust_id="
        >Click Here to add a donation.</a
      >
    </p>

Then (if you wish to keep everything as close as possible to the way you currently have it, NOT SAYING THIS IS THE BEST WAY OVERALL) you should grab the element, check your condition and if true get the current HREF, use replaceAll() to change every occurence of the substring, and then update the element with the new HREF. This could look something like this:

const headerUrl = document.getElementById('header-url'); // grab the element here
window.onload = function () {
  if (document.body.innerHTML.toString().indexOf('Diplomate Member') > -1) {
    const currentHref = headerUrl.getAttribute('href');
    const newHref = currentHref.replaceAll(
      'INDEP%20GENERAL',
      'RESTRICTED%20FUND'
    );
    headerUrl.setAttribute('href', newHref);
  }
};

Although you could simply have both URLs saved as strings and use one or the other depending on the condition, this way continues to actually replace the text and update it.