This option controls whether the due dates of cards will be changed when you enable FSRS, or optimize the parameters. The default is not to reschedule cards: future reviews will use the new scheduling, but there will be no immediate change to your workload. If rescheduling is enabled, the due dates of cards will be changed.
When I do this, my cards due tomorrow are still 203. Is this normal?
Have you changed either parameters or desired retention? I know this sounds obvious, but this feature only works if you change parameters and/or desired retention.