Interesting question and one I am not sure that the contract specifically addresses, but should be able to be dealt with practically. I agree as you say that there should be an clear PMI (instruction) from the Project Manager that the Key Date is no longer required. Not so sure that that would be or needs to be a negative CE though. The works would probably proceed similarly anyway, it is just the extra liability that they are no longer liable for. If this was option C, they may be able to programme the works more effectively which might save cost and increase gainshare for both Parties.
If they did not have to relax the Key Date and they are only doing it for the benefit of the Contractor then that might be one eventuality that a cost saving would be offered which I guess by agreement the easiest way would be as a negative CE. Otherwise I think just the instruction confirming there is no longer a liability to meet the Key Date is all that would be required so both Parties are clear where they stand.
If they did not have to relax the Key Date and they are only doing it for the benefit of the Contractor then that might be one eventuality that a cost saving would be offered which I guess by agreement the easiest way would be as a negative CE. Otherwise I think just the instruction confirming there is no longer a liability to meet the Key Date is all that would be required so both Parties are clear where they stand.