Two methods, use javascript to call window.opener to pass all the changes back to a hidden form on the previous page but I don't like that.
It would be better to treat the two as unrelated once they are opened. I'd submit the form in the little window and print back a "thanks" message with a button which calls javascript's window.close() and that's that.
The original window should be able to see the impact of the changes next time it's refreshed. You could even call window.opener.hiddenform.submit() to call a dummy refresh so long as "hiddenform" exists and has the necessary info.