Regardless of whether approved in Greece, such unilateral contract modification may indeed run afoul of contract or consumer protection laws in the country of residence, country of credit card issuance, and country (USA) in which Marriott is domiciled.

A charge-back or small claims court case may be your only recourse.

If the Marriott Bonv°y reservation system was part of the original transaction, nobody should be giving "internet legal advice" that Marriott is definitively off the hook. Traditional choice of law principles would apply.