How to implement slug in url for a existing route in Razor pages?

I have an e-commerce website already built. In that, the product page route looks like

https://localhost:5000/product/{productId}

I have implemented it in razor pages by using the @page directive as

@page https://localhost:5000/product/{productId}

Now I have spoken to an SEO developer they wanted URL's with SLUG for SEO purposes as shown below

@page https://localhost:5000/product/{SLUG?}/{productId}

To achieve this URL, I have used ? to make an attribute in URL optional and changed the route basically. But the problem is that I have used `<a href="/product/{productId}" in many other places, so I can't change the URL directly with a slug.

so I have decided that all the <a> that uses the previous URL as https://localhost:5000/product/{productId} should be permanent redirected to https://localhost:5000/product/{SLUG?}/{productId}

How can I redirect from one URL to another without adding a separate page and pagemodel to take hit on the route and then redirect?