As I said in my previous post both of them ARE personal aide-de-camps and have been so for decades (5 in Mark's case and 2 in Tim's case): see the dates I posted behind their names. So it would be very hard, or more precisely: impossible, to be appointed to something you have been for a very long time.
The position of personal aide-de-camp is linked to the Sovereign not to whoever occupies that position. Charles' ascension as king only impacted one personal aide-de-camp: Charles himself. He cannot be an ADC(P) to himself.
See wikipedia for more information.