The upgrade() method of the CustomElementRegistry interface upgrades all shadow-containing custom elements in a Node subtree, even before they are connected to the main document.

root

A Node instance with shadow-containing descendant elements to upgrade. If there are no descendant elements that can be upgraded, no error is thrown.

None (undefined).

Taken from the HTML spec:

const el = document.createElement("spider-man");

class SpiderMan extends HTMLElement {}
customElements.define("spider-man", SpiderMan);

console.assert(!(el instanceof SpiderMan)); // not yet upgraded

customElements.upgrade(el);
console.assert(el instanceof SpiderMan); // upgraded!