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I recommend you keep the site as is and just create a shop in a /shop/ subfolder on the main domain. authority comes from external backlinks, if you move the main site to a subdomain (without redirects), traffic will drop and you'll have a ton of links to 404 pages, creating a giant mess. if you do redirect to subdomain, then all of the authority will follow with it (except for links to the main root domain). you'll also need to be careful to not have any urls that overlap with old urls that you are now redirecting (again, high chance of mistakes).