“Shopify's one-click update does this for free.”
It does a lot of it: theme-editor settings come across, and so do code edits that do not conflict with the update. When they conflict, Shopify adds the theme with a note that code edits could not be included. The job is checking which happened, carrying the rest across by hand, and listing it all.
“Why update at all? The old version works.”
It works until you want a section that only exists in the new version. Updating on a copy, with every customization listed, means you choose the moment instead of being pushed into it.
“$349 is more than the theme cost.”
The theme is the easy part. The price covers comparing your theme with its original to find every change anyone made, carrying those changes across, an expert check of home, collection, product, cart and search on mobile and desktop, and a written diff.