Five things actually cause SEO loss during a Shopify migration, in order of impact:
1. Missing 301 redirects (the biggest by far)
Every URL on your old store that was indexed by Google needs a 301 redirect to its Shopify equivalent. Products, categories, blog posts, content pages, image URLs that ranked, even some legacy URLs from earlier site structures. Skip any meaningful chunk of these and you lose the SEO equity associated with those URLs.
This is 80% of the SEO migration story. Get the redirects right and almost nothing else matters. Get them wrong and nothing else can save you.
2. URL structure changes without mapping
Shopify enforces a URL structure: /products/ for products, /collections/ for collections, /pages/ for content, /blogs/ for blog content. Your old platform probably used a different structure. Magento used /category/subcategory/product.html. BigCommerce used /category/product/. WooCommerce used /product-category/product/. Wix used /shop/p/. None of these match Shopify's pattern.
The URL change itself isn't the problem — Google handles URL changes fine when there's a 301 in place. The problem is when the URL changes and no redirect catches it. Same root cause as #1, but a separate failure mode to verify.
3. Lost schema markup
Magento and WordPress stores often had Product, Organization, BreadcrumbList, and Review schema markup that the theme or extensions injected. Shopify themes have varying schema support — Dawn and Sense are good; many paid themes are weaker; older themes may have almost none.
Lost schema means lost rich result eligibility: no review stars in search results, no product pricing snippets, no breadcrumb display. The pages still rank, but click-through rates drop. Often 10-20% of traffic loss post-migration traces back to weaker schema, not lower rankings.
4. Broken internal linking and navigation
Old internal links pointing to old URLs that aren't redirected become 404s within the new store. Theme-level navigation, related product modules, in-content links, blog post cross-references — all need to point at new Shopify URLs.
Internal link rot doesn't hit organic rankings the same way external missing redirects do, but it hurts crawl efficiency and user experience. Google sees a site full of internal 404s and downgrades its quality assessment.
5. Sitemap not resubmitted post-migration
Your old sitemap pointed Google at your old URLs. Post-migration, Google needs the new sitemap so it can crawl the new URLs efficiently. Submit it via Google Search Console immediately at cutover — not a week later, not when you remember. The faster Google sees the new sitemap, the faster the recrawl begins.
The other foundational SEO elements (content quality, backlinks, domain authority, Core Web Vitals) typically don't change much during migration. The destruction patterns are concentrated in these five causes. Address them all and the migration is technically clean.