Changing Permalinks from /%post_id% to /%postname%

Hi,
I have a blog where my permalink structure is /%post_id%
so example: http://www.myblog.com/374

The problem is that the post IDs are always jumping. So every time I write a new post, it’s several numbers more than the previous one. (E.g. after I wrote http://www.myblog.com/374 and then I write a new post, the new post ID might be 386).

What I want to do now is to change the permalink structure to /%postname% so that I can manually put in the numbers for each post.
But I do NOT want to change the permalinks for the already published posts – they should remain the same.

I realize this is probably a unusual request, so any help is much appreciated.
Thank you!