ENGGEN 734 Engineering Contracts for Project Managers

This question relates to how inconsistencies between information supplied by Stewart Limited (the Principal) and information provided by FTC Limited (the Contractor) at the time of tender are subsequently dealt with after contract award.
On our project the Contractor is responsible for the design of the facade of a new office building which will be the Principals new headquarters. The form of contract is NZS3910:2013 Lump Sum. The information provided by the Principal states that the facade must be glass and gives a specific glass reference.
When tendering for the project the Contractor identifies a special offer for a timber panel facade, saving $80,000. He therefore includes in the tender proposals his offer confirmation that the façade will be timber panelled. The Principal does not notice this change and accepts the tender and awards the contract.
The Contractor subsequently submits its design to the Engineer for approval, clearly showing the timber panelled facade. The Engineer notices the discrepancy and discusses it with the Principal, who insists upon the glass facade because team culture is a big driver for the organisation and studies have shown that people are happier when working in spaces saturated in natural light.
a) Can the Principal insist/demand the glass facade based on the agreed contract? b) If the Principal cannot demand the glass facade, what are its options to change the facade? c) How would the Principal/Engineer go about this under the contract?