I am optimizing a site which uses underscores rather than hyphens as word separators (such_as_this.php vs. such-as-this.php). Most of these pages have been around since 2007, and I am hesitant to just redirect to a new page because I am worried it will cause the rankings to slip.

Would you recommend changing the file names to be in hyphenated format and place 301 redirects on the pages with underscores, or stick with the existing pages? Is there anything else that would work better?

Thanks!