Migrating to Shopify, should all link be REDIRECTED in order to keep up with the SEO ?

[Helloweb]

Hi,

I've been advised by an SEO agency to redirect every single URL of my website (more than 5k URL) in order to not loose the SEO work that have been done..I am not quite sure, I've chosen the top 150 URL that are more visited and updated the sitemap...What's your thought ? I am not an SEO expert

Thank you,

Sarah