Moving the catalog is the visible work. Keeping the organic traffic is the work that pays for the project. We treat URL mapping, 301 redirects, metadata, structured data and a post-launch ranking watch as one plan — not a punch-list item for Friday night.
The sequence: crawl the current site; export every live URL that earns traffic or has inbound links; map each one to a Shopify equivalent (collection, product, article, page, or a deliberate gone-page); implement redirects before cut-over; keep titles, descriptions and canonical logic consistent unless you have a reason to rewrite them; move reviews and content that Google already associates with those URLs.
Technical extras that save months: preserve handle logic where it is clean, do not let Shopify’s default /products/ and /collections/ collide with a custom structure you did not plan, and keep a staging sitemap you can diff against production. After launch, monitor Search Console for 404s and query drops for four to six weeks with someone actually assigned to the queue.
Zero-downtime is a cut-over plan, not a slogan: DNS, checkout freeze windows if you need them, OMS and inventory paused or dual-written, and a rollback path. SEO is one track in that plan.
If Magento customisations are doing jobs Shopify cannot, we rebuild those jobs as apps or integrations — pincode checks, ERP sync, wholesale rules — instead of forcing a theme to pretend it is a middleware layer.