Credit card terms and conditions

I was given a copy of your credit card terms and conditions brochure
14.9 Refers to the credit contracts and consumer finance Act 2003
The clause says that the 2003 act applies for credit card accounts established on or after 1st April 2005
Further down it says if the account was established prior to April 2005 then its subject to the 1981 Act (credit contracts), unless BNZ choose (elect) otherwise.

Those two Acts are substantially different so when would BNZ decide on which Act would apply - at the time that the contract with the bank was formed with the customer or afterwards.