Permalink Settings

Pls help to solve the problem
old links are in the format
site.com/2013/04/18/sample-post /
it is necessary not to change them, would remain in this format

But new pages that would be posted later, would be without date in link
site.com / sample-post /

I apologize if you have already asked this question, but I can not find ..
it is important to remove the date from the new links, while do not chage old ones in earlier posts

Thank you for any help you can provide in this situation!