I've modified the timetables on a copy of the database. Will importing the new timetables overwrite any data?

When importing new timetables, you specify the date from which they are to be taken into account. Data relating to lessons prior to this date will not be overwritten.

Homework notebook data for lessons deleted or moved as a result of timetable changes are not overwritten either: PRONOTE offers you the option of attaching them to other lessons when importing new timetables.

However, other data (room reservations, timetable adjustments, etc.) are overwritten.