Permalinks settings changes

Hi, if i have the default wordpress setup for many years on a blog of mine (it has 2500 posts)
It’s now setup like this
http://www.sample.com/?p=123

If i go to edit permalinks, will it do anything to the old link urls?

I want all old urls stay the same.

While all new urls to be:
http://www.sample.com/2008/03/31/sample-post/

like that

Please let me know.

Thank you