Reinstalling the email migration app is simple.
1. Login to admin.google.com (you must be a Workspace administrator).
2. Click Marketplace Apps (or More Controls > Marketplace Apps, at the bottom of the page, if Marketplace Apps isn't already on your main console).
3. Click on the Email Migration app.
4. Click the trash icon in the top right of the page, and remove the App.
5. Install the Email Migration app here.
This process will reinstall the App for all users on the Workspace account.