When creating an etsy style site, how do I 'push' the template site to existing subdomains

Hi

I've been creating an etsy style site, and have got my template site set up which contains the categories allowed for their store.

I know that when I have to add new categories to the template site, existing sites won't get the changes, only new ones from then on, but I heard somewhere there is a way to 'push' the updated template onto the existing store. Can anyone tell me how to do this please?

Thanks

Sally