You may apply to modify your development consent or construction certificate provided that the proposed development as modified is still substantially the same development. Section 4.55 of the Environmental Planning and Assessment Act 1979 stipulates the requirements for modification of consents and Section 148 of the Environmental Planning and Assessment Regulation 2000 stipulates that requirements for the modification of a construction certificate.
If the proposed modification does not meet the requirements of Section 4.55, you may need to lodge a new development application.