Should I do SEO before or after a migration?

Both — and the order matters.

• Before migration: baseline crawl (Screaming Frog), full GSC export, current Lighthouse scores, ranking snapshot for top 100 keywords. This is your “don’t-break-it” reference.
• During migration: preserve URLs, set up 301s for any URL changes, port hreflang and canonicals, port structured data.
• After migration: immediate post-launch crawl, diff against baseline, fix any 404s / redirect chains, resubmit sitemaps to GSC, monitor for index drops over 30 days.

Migrations done without an SEO playbook routinely cause 20–60% organic-traffic drops. Done with one, the loss is zero or near-zero.