Rolling back CompleteFTP is quite easy. The most crucial step is restoring the old version's configuration file.
CompleteFTP uses a configuration file called config.sdf, and it is stored here:
C:\ProgramData\Enterprise Distributed Technologies\Complete FTP
When an upgrade is performed, a backup of the old configuration file is stored here:
C:\ProgramData\Enterprise Distributed Technologies\Complete FTP\Backup
Backups are named by their version number, and are called either daily_[version]_[date].sdf or update_[version]_[date].sdf.
To roll back to a specific version, you need the most recent configuration file for this version,. e.g. to roll back to 9.1.3 from 10.0.0, you need to locate the most recent version of the 9.1.3 config file. For the installation on the author's machine, that file is update_913_20161222.sdf.
Once this file is located, follow these steps:
1. Uninstall CompleteFTP (it won't remove backups).
2. Copy the previous version's configuration file (e.g. update_913_20161222.sdf) to the configuration directory (C:\ProgramData\Enterprise Distributed Technologies\Complete FTP), renaming it to config.sdf.
3. Install CompleteFTP using the rollback version's installer (obtainable from our customer site).