You can change the Appointment's duration while dragging inside of the CanDrop
method in the custom DragDropBehavior. The method is called multiple time during the dragging and can check inside if the Appointment has reached a Slot. However the desired scenario can be only achieved by using a regular SpecialSlots not a ReadOnly ones as when a dragged Appointment goes above a ReadOnly Slot dropping is disabled and the visual dropping target disappears. I'm attaching a sample project which demonstrates how the Appointment can be resized when dragged above a SpecialSlot.
Hope this helps.
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