Disclosure: this is from GTP-5 but I've tested it in a browser

replaceClassNames.js

/**
 * Replace text inside class attributes across the DOM (allows partial matches).
 *
 * @param {string} find - Text to find inside class attribute (partial allowed).
 * @param {string} replaceWith - Replacement text.
 * @param {Object} [options]
 * @param {HTMLElement|Document|Element} [options.scope=document] - Limit to a subtree.
 * @returns {number} Count of elements that were modified.
 */
function replaceClassNames(find, replaceWith, { scope = document } = {}) {
  if (typeof find !== 'string' || find.length === 0) return 0;

  // Escape regex metacharacters in `find`.
  const escapeRegExp = s => s.replace(/[.*+?^${}()|[\]\\]/g, '\\$&');
  const re = new RegExp(escapeRegExp(find), 'g');

  let changed = 0;

  scope.querySelectorAll('[class]').forEach(el => {
    // Always read from the attribute so it works for SVG (className may be an object).
    const before = el.getAttribute('class');
    if (typeof before !== 'string' || before.length === 0) return;

    // Partial replace within the full class attribute string.
    let after = before.replace(re, replaceWith);

    if (after !== before) {
      // Normalize whitespace and dedupe class tokens.
      after = after.trim().replace(/\s+/g, ' ');
      const deduped = Array.from(new Set(after.split(/\s+/).filter(Boolean))).join(' ');

      // Always write back via attribute to keep behavior consistent across HTML/SVG.
      el.setAttribute('class', deduped);
      changed++;
    }
  });

  return changed;
}