The change to a class schedule only affects the dates after the change.
I noticed when a schedule had changed from like Thursday 4PM to Thursday 2PM. The Attendance sheet shown the change across the entire semester and at first it seemed like the attendance data was lost but it just was not being displayed because no attendance had been recorded for those specified times in the past. If there were a way to keep the old dates/times up to the point the change in the schedule occurred that would be awesome. If not at least be able to export the "Attendance" data.
That’s the way it works. It basically update the schedule for the whole semester, but in the case you already have recorded attendance, it won’t change (because you don’t want to lose that info!).