changing urls to dashes from underscores

I have a large site w/ over 9,000 pages indexed in google. How can i safely change my urls to (-) dashes vs (_) underscores ? Ik now how to do it in the backend , i just want to know if all my links will be broken afterwords.

???

If you change any settings in SEF Advance afterworlds your links might be broken, so it's not recommended. The settings are to be set when you start the site and not changed later. You could setup automatic redirect in .htaccess with mod_rewrite but still you would use ranking so better don't touch the settings when site is already indexed with many URLs.