Here a brief explanation of what happens when a media campaign gets changed or cancelled:
1. If your Credit Note Media Campaign has not been fully invoiced but only partially for a certain time period (i.e. monthly: January) and you change something (6.4 Cancellations & Credit Notes / 4.11 Optimizer), then these changes get added to the pen Credit Note Item (i.e for February).
2. If all Credit Notes for your Publisher Payout are invoiced - any changes will be lost and not considered in a new credit note.