What is the latest I can edit my client's regular premium?

Both new Direct Debits and amendments to existing Direct Debits take 8 working days for the AUDDIS (Automated Direct Debit Instruction Service) to be set up/amended. Any changes to a direct debit within 8 working days of the collection date will not apply to the in-progress direct debit. The changes will be reflected in the following month’s direct debit payment.