Migrating a shopwithout losing SEO.
The usual failed migration is not a case of the new shop being worse. It is a case of the old addresses ceasing to exist while Google spends weeks walking into a wall.
This text is about one thing: keeping your addresses. The rest of a migration, moving products, orders and customers, is technical work you can redo. Lost search positions are not as easy to undo, because rebuilding Google's trust takes months rather than hours.
What exactly breaks
Every platform builds addresses its own way. Category endings change, so does the way variants are written, the addresses of your terms pages, sometimes the whole structure. If an old address stops answering, a visitor from Google gets an error page, and after a few such visits the search engine drops the address from its index. You lose not only the position but every link that has pointed at that address from outside over the years.
The redirect map, the only thing that really counts
Before the migration, build a full list of the addresses that currently have traffic or links. Take it from three places at once: the old shop's sitemap, Google Search Console and your analytics tool. Against every old address write the new one. Mark the addresses with no counterpart deliberately too: a product you no longer sell should lead to its category, not to the home page, because to Google the home page is the answer "I do not have this".
Redirects have to be permanent, that is code 301, not temporary. Only a permanent redirect carries to the new address the value the old one gathered over years. Do them in bulk from a file rather than by hand: with a few thousand products, manual work guarantees typos. In Torveo there is an import of 301 redirects from old addresses for exactly this, so you upload the map as a file and check the result before going live.
The first week after launch
On switch-over day, check a dozen or so addresses from the map by hand, including your most popular products and categories. Then watch the indexing error report: the odd 404 is normal, but an avalanche of them means the map has a hole. Do not change two things at once during a migration, if you change the category structure or product names along with the platform, you will not know what hurt you. Introduce changes one at a time, a few weeks apart.
A migration without losing traffic is achievable and it is not magic. It takes one boring thing done properly before launch rather than after it: a complete map of old addresses to new ones.