You can manually move processes from the TS scheduling class into the FSS scheduling class without changing the default scheduling class and rebooting.
Become superuser.
Move the init process (pid 1) into the FSS scheduling class.
# priocntl -s -c FSS -i pid 1 |
Move all processes from the TS scheduling class into the FSS scheduling class.
# priocntl -s -c FSS -i class TS |
All processes again run in the TS scheduling class after reboot.