Change dates on multiple posts at once?

I have a website on which “subscriber” (password protected) content rolls over into a public (free) section after a few months. I have to do this manually, converting a batch of 18 posts every month. I’m wondering if there’s a plugin which would allow me to modify the post dates of all of them at once (to the date they are “reposted” in the public area). As is, I have to do each one manually, which takes forever. The “quick edit” feature of the “edit posts” page won’t allow this.