The fluent API should make this pretty easy. Just iterate through all the pages and change the name using ToLower()
I posted an example of how you can use the API to change the sitefinity page extensions
on my blog, Doing something like
page.UrlName = page.UrlName.ToLower()
should work. You can execute this on a dummy .aspx webforms page codebehind.
Just be sure to:
1. Backup your site and db first (just in case!)
2. Let me know if this doesn't work so I can help further!
hope this is helpful!