Thanks for everyone's feedback. You can now easily extend or upgrade a booking. 

You will now see a new drop-down menu in a guests booking. 

Extension/upgrade - use this to process an upgrade or extend a booking. You can select a different agent from the one that originally made the booking. So a guest that may have booked through booking.com wants to extend their stay directly with you, by selecting agent as 'direct' the cost of the extension won't be included in the booking.com commission calculation.