replaceElement

Use p.parentNode.replaceChild(newChild,oldChild)

  const codes = qq("pre code.language-mrflibble")
  for( const code of codes ) {
    const p = code.parentElement
    const div = document.createElement("div")
    div.innerHTML = code.innerHTML
    p.parentNode.replaceChild(div,p)
    div.classList.add("hexvision")
  }