NEC ECC: Proposed alternative piling method takes longer that what is shown in the Accepted Programme

Type of Contract: NEC3 ECC Option A
The Contractor has sent out some inquiries for some subcontracted piling works, which will be Contractor-designed or Subcontractor-designed to meet a performance specification in the Works Information provided by the Employer. The Contractor is advised by one tendering SubContractor that some alternative piling could be provided at half the price allowed for in the tender and which meets the performance specification for the piling in the Works Information. The alternative piling will however take longer to complete then what is shown in the Accepted Programme.

Can the Contractor change the piling method that the pricing was based upon and shown in the Accepted Programme?
How is this implemented under the contract?

I believe this is an exam question for the NEC Project Manager accreditation.