Так что просто переберите все элементы, которые вам нужны, и выполните замену только этих элементов.
// Get all the elements that you care about into an array
let elements = Array.prototype.slice.call(document.querySelectorAll("p, header"));
// Loop over the items in the array
elements.forEach(function(el){
// Do the replace on the element
el.textContent = el.textContent.replace(/text/g, "letters");
});
<header>This is some text in a header</header>
<h1>This is some text in an h1</h1>
<div>This is some text in a div</div>
<p>This is some text in a p</p>
<div>This is some text in a div</div>
<div>This is some text in a div
<p>This text is inside of a p with lots of text</p>
</div>