[PeterisP]

I've been a part of multiple "big bang" migrations in banking (generally scheduled to the coincidences of local banking holidays next to a weekend, so you can afford multiple days of semi-downtime) and all of them had explicit multiple testing gates for potential rollback during the migration, where after pretty much all the stuff is on the new system, the board convenes and after looking at the difficulties (there inevitably are some unexpected difficulties) make the decision whether they "accept" the switch to the new system or postpone the switch.

Part of your preparation and testing is the rollback of a partial migration - if you're irreversibly committed to the "big bang" before you know its outcome, then your preparation and testing has failed.