Hi there,
The URL structure in Shopify can't be changed as it was fixed.
With 5 years experience with Shopify, I understand your issues as you moved your store from another platform to Shopify. But the URLs structure is a little bit different, and you want to keep the SEO ranking.
As you can't (and nobody can do it) the URLs in Shopify, I have a solution for you. It's redirect your old URLs to the corrrect new URLs in Shopify and it helps to keep your ranking.
For example, your old URLs structure is: [login to view URL]
But now this url is not available and it caused 404 not found error. Thing you need to do is redirect [login to view URL] to [login to view URL]
It will help to keep your SEO ranking and solve the 404 not found errors.
I can help to make it work for you.
Contact me for more advise.
Regards,
Tran