SIMS: Running a sync when you have an open parents evening
If you need to update your data you can run the SIMS Extract Tool whilst an evening is open. Doing so will not affect existing bookings unless you untick something (year, teacher, class, etc) previously ticked when running the Extract Tool - and even then it will warn you about this before you get to the final stage and give a preview of what would be deleted, so you can identify if you need to go back and change the sync before completing it.
We also recommend quickly checking Configure Evening > Class Teachers for the evening, as anything on that page which is set to default (i.e. the row is white rather than blue) will always show the name of the default teacher in SIMS for that class at the time of the most recent sync and (although it's unlikely) this may have changed in SIMS since you created the evening.