Can I freely change the permanent deadlines?

When installing your database, a list of permanent due dates was created.

You can modify all permanent due dates at your convenience. This is your database, filled when installing your account with default data, and that you can change at your convenience.

You can keep due dates that you do not currently use but could use. You can delete them if this is more appropriate for your ease of management.

And you can add lines, and modify existing lines at your convenience.

If you modify a due date, it will be modified at the scale of your database, and this will therefore have an impact on all customers who are associated with this due date (already associated, or associated in the future).