There are multiple reasons, and almost all relate to incompatable releases.
You cannot just go from an 071 to an 072 for example.
You can't load the firmware from the wrong device (A 7835 and a 7845 do not actually have the same firmware.)
Tell me what firmware is running now and I will know the device and can tell you whatever upgrade path is needed to get it to the latest 072 release, or even the 073 release (It will change the way "Workflow Scanning" looks though in the 073)