The client changed the advanced payment clause one month after signature of the original contract. The project is a 4 month project.
Clause 12.3 states that that unless a change is provided for by the conditions of contract, changes can only be made by agreement and confirmed in writing by the Parties.
Therefore, unless there is a relevant Z clause, the Parties would both have to agree to the change ie the Employer and the Contractor. If there is no agreement then the change can not be made.
Yes, but only if the other Parties to the contract agree to the changes. See Core Clause 12.3 of the NEC3 ECC Contract.