I have been involved in two core banking system replacements, both were successful and both were basically Big Bang conversions.
The phased approach is superficially attractive, but falls down for various reasons.
The main problem is that a large percentage of transactions are between a banks own customers. Having to deal with transfers between a customer in the old system and a customer in the new system, requires a whole new set of business rules, special coding and nearly doubles the use cases to be tested. The only real advantage is that phased conversions are usually quietly dropped half way through as the workload increases and the budgets get blown leaving the customers blissfully unaware of the chaos in the IT department.