Migration: Plus → AC, AC → Plus, timeline + cost?

Both directions are well-trodden. Realistic enterprise numbers:

Shopify Plus → Adobe Commerce (most common at the $25M+ B2B band):

• Timeline: 9-15 months from contract to DNS flip. Catalog migration 4-6 weeks, B2B re-architecture 6-10 weeks, ERP / OMS / PIM re-integration 8-12 weeks, theme + checkout build 12-20 weeks, parallel-run + redirects 4-8 weeks.
• Cost: $300k, $1M for the build + $100-250k for ERP / OMS re-integration + $50-150k for first-year ops retainer. Total Year-1 outlay: $450k, $1.4M.
• Risk: URL redirects miss long-tail pages (drop SEO 15-30% for 6-8 weeks if redirects are sloppy); Shopify customer passwords don’t migrate (force reset); Shopify-app data (Klaviyo events, Yotpo reviews, Loop subscriptions) needs a separate migration plan. Subscription customers often need re-tokenization at the gateway.

Adobe Commerce → Shopify Plus (common when an AC store has outgrown its dev capacity):

• Timeline: 6-10 months. Faster than the reverse because Plus is more opinionated, less custom architecture to rebuild.
• Cost: $150k, $500k for the build + $50-150k for ERP re-integration via middleware + Plus license year-1.
• Risk: AC custom logic (Magento_Sales overrides, complex price rules, custom B2B workflow) often doesn’t fit Functions, re-scope or accept reduced functionality. Multi-store catalogs collapse to a single Markets store or expansion-store cluster.

Run both stores in parallel for 30-60 days behind a feature flag. Flip DNS only when the redirect map covers ≥98% of traffic, 50+ test orders pass on the new platform, and the support team has handled 5+ real customer issues on the new stack.