Of course you can manually change them to lower case. If you want the change to be automatic when you create a new product you will need to use JavaScript.
Personally, I don't see any seo benefit or hindrance using uppercase.
Can you explain what you are trying to accomplish exactly? Are you wanting lowercase to be automatic when creating a URL, or are you wanting to rewrite all your URLS to lowercase?
http://www.seoverflow.com/redirect-uppercase-urls-to-lowercase/