As I understand 63.13 it provides the mechanism to record the CE. You still use the forecast of Defined Cost but, having done that, 63.13 provides that the result is pushed back into, or recorded in, the BoQ. This is because the interim payment under of PWDD is by reference to quantity and rate in the BoQ.
If you have 100m of trench at £10/lm in the original BoQ and then a further 100m is instructed and you assess that to be £15/lm you would need to adjust the BoQ either to provide the new trenching as a new item (because it is sufficiently different from the original and that is what is driving the price change) or within the existing item by combing the original and new so 100 * 10 = £1000 plus 100 * £15 = £1500 so 200 = £2,500 so the new linear rate would be £12.50 for trenching.
That is at least a lawyers answer, a QS may have a more sophisticated/different apporach