[w3c/webcomponents] Updating Element Registration. (#820)

Are there any discussions to support updating the implementation of a custom element definition? (Searched and did not spot anything obvious)

Something like this:
```javascript
customElements.define('my-element', MyElement);
// ...
// some time later...
customElements.redefine('my-element', MyElement2);
```

I can see a lot of issues with this, the obvious one being what happens to existing elements that are already in the DOM - are they recreated? left alone with old implementation? Or just turned into no-ops? 

### Use case

Let's say, I have a web component that is sitting on a page with long life time .- say email client or stock dashboard which people have open all the time and rarely 'refresh'. 

The client code at some point realized there's a new version of `my-element` available and I want to upgrade it to the latest without reloading the whole page. 

This is particularly useful if there's some sort of change in the protocol/api the custom element may use with the back end. 


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