When the euro first becomes the monetary unit of participating member states of the European Union, a reference to the ECU in a contract, security, or instrument that also refers to the definition of the ECU in Section 5 shall be replaced with a reference to the euro at a rate of one euro to one ECU. A reference to “ECU” in a contract, security, or instrument without defining “ECU” shall carry a presumption, rebuttable by a showing of the contrary intention of the parties, that it is a reference to the currency basket that is from time to time used as the unit of account of the European Community.