Will I lose any data, customer accounts, or order history?

No. Magento upgrades are schema-only, your products, customers, orders, reviews, CMS pages, configurations all stay intact. We snapshot the production database before every step, run the upgrade on a clone, validate counts (orders, customers, products) match before and after, and only then promote to production. Plus the blue-green setup means the original DB is preserved for 72 hours.