I am using Debian Testing as samba file server to share files to windows clients. Previously, "date accessed" stamps were helpful in cleaning up the folders on the file server. If a file hasn't been accessed in 3 months, I can move it to an archive folder or something.

I recently installed Unison and used it to keep the file server in sync with another file server. Cool.
Unison touched every file on the file server and changed its "date accessed" time stamp to the current date. Oops.

Is there a way to change back to the time stamps before this happened?

Short answer: NO.
Once you do touch on a file, its timestamp is changed. That's it, unless you have a backup. And that is the way it should be.
Sorry.

You could use the mtime instead.

Good point.
I'll remember that.
I learn something new every day. Thank you!
(Hope you find that job soon)