To your closing question: I'd avoid "sign up again" as the default and offer a slower verified route instead. A few checks I'd accept when the old inbox is gone: (1) confirmation from another admin or the billing contact on a team account, (2) details only the owner would know, like the name of a recent project and roughly the last thing they exported, and (3) a short video call where they log in from their usual device. Give those the same 24-hour delay you suggest. One pitfall: don't let support decide case by case. Write a one-page checklist that needs at least two matching signals, and log which ones were met, so the answer is the same whoever picks up the ticket.