Using search and replace queries, I've changed the underscores in our URLs to hyphens.
It's my understanding that Google may prefer hyphens to underscores. Also, hyphens are easier to speak, read and type.
I've modified the PHP code so that the old URLs (with underscores) still work. There are plenty of these old URLs stored in Google and other search engines.
The MySQL query syntax for search and replace is pretty easy:
UPDATE table_name SET field = REPLACE(field,'_','-');
— posted by B on May 29th, 2004