Can I migrate from self-hosted Magento to ACC without downtime?

Yes, with proper planning. The pattern: (1) spin up ACC Integration with a full database dump from your current production, (2) validate functionality on Integration + Staging, (3) set up real-time DB replication from old → new for the cutover window, (4) off-peak DNS swap with TTL pre-lowered to 60s, (5) keep old system warm for 72 hours as roll-back. Total downtime: typically 0-30 seconds during the DNS flip. Order data flows to new system immediately.