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.