Run the diff before the debate
Download the live theme and compare it against a clean copy of the same version from the theme store. If the diff runs to a handful of files, you have a maintainable theme. If it touches dozens of files and theme.liquid is unrecognisable, upgrades are already impossible.
That single check settles most arguments. A locked theme means every future fix is bespoke and every app install is a risk. That is the real cost of keeping it, and it compounds every quarter.
Rebuilds fail on data more often than design
The design is the easy part. Redirects, metafields, collection rules, subscription contracts, review data and third party integrations are what break launches. Map every URL, every metafield namespace and every app dependency before anyone opens Figma.
Budget realistically. A UK re-theme on an existing Shopify store typically lands in a wide range from around £10,000 for a themed build to £50,000 and beyond for heavy custom work. Treat any quote outside that range as a question worth asking.
Sequence a rebuild so revenue never dips
Launch on the same URLs, with the same product handles, in a quiet trading week. Keep the old theme published until the new one passes a full checkout test on iOS, Android and desktop. Shopify lets you preview an unpublished theme against live data, so use it.
Expect a short dip in organic traffic if URLs or internal linking change. Hold the old sitemap in Search Console, watch Core Web Vitals for four weeks and keep a rollback theme ready for thirty days after launch.