> The F5 load balancer (hardware commodity) could've slowly cut over the traffic 10% at a time to the new migration site to get a feel for user experience.

I doubt an F5 load balancer would work in this specific case. But there definitely should've been a software router-adapter that routed requests to two systems and converted their replies to a single format. This would've let them migrate their customers in batches instead of a big bang cutover.

That's what I've always done when migrating data to a new banking system.