If you've used templates correctly, you should only need to make the modification on one of the pages and the others will be updated as well.
In most cases a news list/detail like that uses a simple page with a repeater that has a list transformation and a detail transformation. In this example the "detail" inherits the parent page template so if you update your list page and your transformations you should be good.
Now not knowing your setup, your scenario could be different and each news archive has it's own template or archive. If that's the case, I'd suggest creating a new setup as I described as it will take you a shorter amount of time than running through 500+ pages to update.