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Couple of things - 1. Have you redirected from original website to the BigCommerce site and also redirected the shopify links to BigCommerce? The last thing you want is a chain like this - original > Shopify > BigCommerce 2. How long ago was this change? If this wasnt long ago then its probably best to wait for it get back up to the same/similar rankings as your original website. If you have done your redirects well, it should take a few weeks and the rankings should get back to normal. 3. Was there a structural and content change to your pages? If the answer is yes then its not really a straightforward migration. This is a complete new website you have got now that is not just a changed platform but different architecture and structure and maybe content. In this situation, its fair to say that there will be a lot of ranking changes. Was your original ecommerce website was handcoded by you and with shopify you used their templates? 4. As for 404, if there is no organic traffic going to these pages then you dont have to redirect them. You may have to spend some time looking at Google Analytics A lot of times this happens when you switch from one platform to another, there is an impact on the organic rankings. There would be something performing differently which can impact your sites visibility. Like Jordan mentioned, I would also suggest that you should focus on improving the site speed. Malika