Looking back on that giant rewrite project, that's how I'd have done it... I'd have built the new system in parallel with the old one, not sharing the data stor

Looking back on that giant rewrite project, that's how I'd have done it... I'd have built the new system in parallel with the old one, not sharing the data store. The new system would have significant advantages over the old system (ie near-realtime transactions rather than waiting for overnight batch jobs). Get it running, and encourage early-adopter customers to switch over. That will stress-test it and allow it to scale. After a few years, with lots of warning, retire the old system. That gets away from the "Flip a switch on billions of dollars of transactions a day" terror.