The answer is what ever your contract states is the answer. The fact you are using a schedule of rates to build up compensation events suggests those are the written rules, and the same would be applicable for de-scope i.e. you use pre-agreed rates for additions OR omissions.
CECA rates in my experience are traditionally chosen for equipment rates to be used when building up compensation events rather than materials and people cost but it depends on what your contract says.
Normally under NEC contracts you build up from first principles in accordance with the schedule of cost components. For example, if you are instructed to install additional steel work then the CE costs would include the reasonable market value of steel at that point, rather than have a pre-agreed rate. This takes away the risk of the Contractor to have to price for increases in materials for events that they have got no idea if they could be instructed. If there is an amended contract to say it will be assessed using pre-agreed rates instead of elements of the schedule of cost components then that is the rules that the Contractor is pricing for at tender stage and entering into by signing up to that contract.