Well, You'll loose all your index threads in mysite.com/forums. Other than that factor, it's really easy. Just change the URL in the ACP then create the subdomain.
All the links were manually typed, so unfortunately that would be the case And I'm not so sure about that script... I have one that brings people back to the forum home if the page is not found, but you would probably need to know some extensive coding to get what you want working...