How to update sites that were made using Blog Templates?s.

I'm using Blog Templates on my multisite. Several blog/sites were created using the templates.

Several functionality changes and bug fixes have been made to the theme that the template was made from. How can I update the older blog/sites with the themes new changes and fixes?

Even if I need to do it manually where are the references to the old theme (which no longer exists on the site) that the old blogs are using.

For example the old blog/sites, if you look at a source page has html that is no longer even in the updated theme. Where is it loading from? Is it in the database?

Thanks