Thanks Jon(and thanks for pointing out) - yes I need to caveat my answer which I slightly rushed. It also depends now if you are on NEC3 or NEC4.
NEC3 first of all. It depends if this is an Employer designed scheme or Contractor design. My answer assumed it was an employer designed scheme, and if so then the total of the Prices CAN reduce. If it was a Contractor designed scheme, then as Jon pointed out the Prices can’t be changed and under option A that would be their Contractor benefit and under option C it would be shared benefit.
NEC4 now changes the rules. It allows the Contractor to suggest value engineering even if it is an Client designed scheme and the total of the Prices under option A would be decreased by an agreed value engineering ratio, and under option C sorted out in the gain/painshare.
What cant change under NEC3 or NEC4 is the Completion date moving forward. That can only be done through acceleration - and that has to be by agreement.