Backfill join logic billing error

A migration added a is_active BOOLEAN NOT NULL DEFAULT true column to a 50M-row subscriptions table and ran a backfill to set is_active=false for cancelled subscriptions using a join against a cancellations table. The migration 'succeeded'. An hour later, billing runs and charges ~12,000 customers who had cancelled. Dashboards: refund/dispute tickets spike; the new column shows almost everyone as is_active=true. Investigation hint: the backfill query used a LEFT JOIN and a WHERE cancellations.id IS NOT NULL that was negated incorrectly, plus the cancellations table uses customer_id while the join was on subscription_id. How do you triage, stop wrong charges, and correct the data?