What happens as part of the change?
On December 1, 2020 (starting 8 p.m. CET) the current FileSender test system https://filesender-test.aco.net/ will become available at https://filesender.aco.net/, taking over from the previous/old system.
Between 8 p.m. and midnight that day neither system may be availble, so plan your FileSending activities accordingly. 
The system previously available at https://filesender.aco.net/ will be renamed to https://filesender-old.aco.net/ and will remain accessible there for a few days so that all previously uploaded files can continue to be downloaded (until they expire, as usual). There will be no need (or possibility) to log in to filesender-old.aco.net and no new files can be uploaded there. Once all uploaded files have expired on filesender-old.aco.net the system will be removed.
What happens to my data uploaded before the change?
All files uploaded to the current system will remain available for download at https://filesender-old.aco.net/. Any "Download Links" you may have shared with others will therefore continue to work fine (until they expire): These will automatically be rewritten to https://filesender-old.aco.net/ and can be downloaded from there. So all data shared with the previous system will continue to be available for download, until it expires (as usual).
What happens to "Guest vouchers" I have issued from the old system?
Invitations to upload files issued from the old system will no longer work after the old system has been renamed (and no new files can be uploaded there after the change). Instead you can issue and share new invitations from the new system.
What happens to my data uploaded to the test system?
Files uploaded to https://filesender-test.aco.net/ before the change remain available after the change. The server and all data will then be accessible via both https://filesender.aco.net/ as well as via the previous/current name https://filesender-test.aco.net/. All files and all download links will remain valid and will continue to work.
(At some point after the change the server https://filesender-test.aco.net/ will be removed so only the well-known production system URL https://filesender.aco.net/ should ever be used/accessed/stored in bookmarks/etc.)
Was happens to "Guest vouchers" I have issued from the test system?
Those should remain fully functional until they expire. Uploading files using these vouchers will happen to the new production system.
What about the FileSender Python client?
After the FileSender server change completes please re-download the latest version of the FileSender "Python CLI client" from the new production FileSender web site ("My profile") to update the server name encoded into the client. Otherwise the client will fail to connect once filesender-test.aco.net no longer points to filesender.aco.net.
Any generated "API Secrets" will remain valid and usable before and after the server name change.
I still have other questions not answered here!
Please feel free to contact the ACOnet Team via e-mail at filesender@aco.net.