Samsung Knox E-FOTA does not support firmware downgrades. If you use Knox E-FOTA to perform a forced update on a device that has a newer firmware version than that of the update, the device's firmware is not updated.
In some scenarios, a device might go into an abnormal state. If this happens, you can try the following: Let's say you forced a firmware update from version A to version B, but a device already had a later version C. If the device with version C gets into an abnormal state, force a firmware update on that device to version C.
One way to avoid scenarios like this is to query the firmware version on a device (using Android Build.VERSION) first, and make sure that the device does not have a later version before you force an update on the device.