For an example of how this system switchover looks like, First Bank in the United States just did a whole-system switchover in March, at least for their business customers. They handle my wife's business' banking, which means I do all the paperwork, and it definitely was a pain. Basically it was like signing up for a whole new bank account. They copied over something like only the last 6 months of transactions and some payroll info, but things like automatic bill payments and everything else (even basic online login) had to be set up again.