When Shopify updates a customized theme, it may report that some code changes could not be included—but it usually does not explain which customizations are actually missing.
The usual options are to compare Liquid, JSON, CSS and JavaScript files manually, hire a developer before knowing the actual scope, or remain on an outdated theme.
I built a Shopify Theme Update Audit for this exact step.
You upload:
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your previous customized theme ZIP;
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the new or updated theme ZIP.
The automated report shows which detected customizations are present in the new theme, which are missing, which appear to be only partially transferred, where conflicts exist, and which areas need particular attention.
One audit costs €49. No Shopify admin access or store login is required.
The audit does not modify the theme or transfer the code automatically. It gives you or your developer a clear migration checklist before further work begins.