Perhaps I am just missing something obvious but how can site scripts (newly emerging SharePoint site-template system) be applied to an existing site? None of the Microsoft help pages/guides seem to demonstrate that, though at least one says:
"Site designs are like a template. They can be used each time a new site is created to apply a consistent set of actions. They can also be applied to existing modern sites (Group-connected Team and Communication)."
Running a site design on an existing SharePoint site processes delta changes. This means that either changes apply to the site without corrupting existing data or that a full provision occurs. Of course, the full provision will occur if the provisioned data hasn't been available in the site before.
This can only be done programmatically since there is no UI support yet. The PowerShell script below demonstrates to you how to run the site design on an existing site.