In short, the answer is 'No' as you would be making changes to the Contract Data which neither the Employer or Project Manager has the contractual authority to agree to.
If you do want to do it, it would be by Supplementary Agreement between the Employer and Contractor of which, part of the agreement, would almost certainly include a rise in the Prices over and above what would be in a compensation event quotation alone for the extra works (and no sectional Completion).
If you do want to do it, it would be by Supplementary Agreement between the Employer and Contractor of which, part of the agreement, would almost certainly include a rise in the Prices over and above what would be in a compensation event quotation alone for the extra works (and no sectional Completion).