In my experience, when you uninstall any AV it needs a reboot. Sometimes the av uninstallation takes longer time to uninstall while you have set another av to install it after that. Of course there are many ways to achieve this task. here is my simple question, why are you not breaking this process in two steps.
1) Uninstall fsecure cleanly on all machines.
2) Install the new AV on all machines.