If a compensation event has been agreed and payed can the Project Manager then at the end of the contract revisit the compensation event and change his response
If you mean an “implemented” compensation event then no.
Implemented is when the Project Manager has accepted the Contractor’s quote, made their own assessment or it is treated as having been accepted (cl. 65.1). This like other communications under the contract should have been confirmed in writing (13.1) and separate from any other communication (13.7). The only part of a CE that can be revisited is when the Contractor has based the quotation upon stated Project Manager assumptions(61.6) that turned out not to be correct.
The only way that either party can challenge an implemented compensation event is through adjudication.