I am not sure how you are administering this matter under the contract, although I presume that the Contractor's proposal was formally confirmed as an instruction by the PM to change the Works Information.
If the Contractor is undertaking the design, this would not be a compensation event, see 2nd bullet point under clause 60.1 (1), so there would be no need for a consequent quotation, unless your conditions of contract have been amended to include a procedure for 'value engineering', such as clause 16 under NEC4 (Contractor's proposals). The Prices would not be amended, although under main options C and D the PWDD could be increased, with this addressed under the share calculation.
Under the ECC form, if the Contractor is not undertaking the design and is putting forward a proposal anyway, with main options A and B this matter would be treated as a 'routine' compensation event and the associated procedures. .With main options C and D the 1st bullet point of clause 63.11 provides that a Contractor proposal is excluded from any reduction of the Prices, although a quotation may be submitted which increases both the Prices and extends the time for Completion.
Ideally the matter should have been dealt with as a proposed instruction, with a submitted quotation which would have influenced the decision to proceed, or not as the case may be.