I've seen a few options here, none of them perfect. 

1. You could just rename the url suffix of the new portal to match that of the old portal so that the links are still valid.
2. If you're using different pages you can use page route maps to route from the old page to the new page.
3. If the pages are the same and it's just the service portals that are different you could write some client side js in a widget and add it to all of your pages that looks at the url onload and then redirects the user if they're using the old portal. Unfortunately I don't have any examples here.