This has already happened. The UK banks recently updated their terms and conditions to include clauses where you the customer effectively wave your data protection rights. This allows them to transfer data abroad and give it to foreign tax and legal authorities.
Currently, for the UK at least, a foreign tax authority must go through HMRC to obtain financial information about you. For this to happen, the UK must sign an IGA to transfer the data. The banks are worried that the IGAs could be declared unlawful through a court action. So the banks have decided to hedge their bets by updating the terms and conditions of the banking services so that all customers wave their data protection rights.