Changed single permalink but nested posts still contain old link

I changed a permalink only on one specific page where it says you can ‘edit’ that page’s permalink. I did this because the name of the page didn’t make sense to customers. But now everything that was posted within that page still holds the old permalink name. So:

It was: http://www.example.com/forum

I had posts inside of that automatically nest like http://www.example.com/forum/10-22-2012

I changed the page from “forum” to “discussion” so it says http://www.example.com/discussion

But when I go inside the page to posts, it still says http://www.example.com/forum/10-22-2012

Is there a way to rewrite the whole structure including its nested subjects?