Can the value of an implemented CE be changed by agreement between the parties if it is subsequently determined that the agreed amount was wrong and both parties agree that is the case.
There is no express provision allowing this.
However, it is implied in the contract because there is a compensation event for a changed decision by the Project Manager or the Supervisor - see clause 60.1(8). Note that it only allows for the Prices to go up and no compensation event can bring forward (sectional) Completion Date(s).
If this was what was desired then, strictly by the contract, you would need a Supplementary Agreement to reduce the Prices & Completion Date(s).