Slashes get stripped out of custum permalink/slug

I want to change my permalink structure from
/%year%/%postname%-%post_id%
to
/%postname%/

I want to keep my old posts for some reason native with the old permalink structure and only want to use the new structure for posts after the change. So I dont want to use a 301 redirect for my old posts to the new urls.

I thought I could simple edit my post urls in the post editor, but noticed if I try to change the Post URL from
my-post-title
back to
2011/my-post-title-3214

the slash after the year 2011 gets stripped out the URL after saving. Is there any way how I can use the slash in the custom permalink URL and rebuild my old URL structure for my old posts manually?