What's an SEO Migration?
It keeps the free traffic you get from Google (technical term is SEO or Organic traffic).
If you move eCommerce platforms i.e. Shopify to WooCommerce all the page addresses change (www.yourwebsite.com.au/shop/productname)
What happens if you move home and don’t redirect your post? You miss important bills or letters.
It’s the same when you move website platform.
You have to implement something called an SEO Migration.
It’s a big subject which has been covered well by many others so here are some links to explain what you need to know.