I've worked on zSeries machines running S/360 binaries from the early 70s. It is one of the primary reasons banking and related sectors that are heavily regulated stick with such platforms for core accounting.

The single whole-bank migration I was involved with was zSeries to zSeries.

This is where the challenger banks really have an advantage. They're not saddled with the baggage of an overnight batch run of JCL decks and Data Sets to arrive at what their regulators consider the reconciled position of the bank.