You'll need to create a redirect from the old URL to the new one ( you can use Easy Redirect app:

https://apps.shopify.com/easyredirects)

2. Afterward update the inventory once notifications are sent out make the product unavailable.

3. Check notifications are sent

4. Shopify only allows redirecting on products that are inactive URLs (404). remember it will not work if the product is available, so make it unavailable on the store.

So once the customer receives the email that the product is in-stock they click on it they will be redirected to the new product.