If I Purchase the Paid Version of a Theme, Does It Get Added to My Current Free Version?

HubSpot does not have a feature for merging different versions of themes. When you purchase a paid version, you buy a separate theme from the free version and must configure all the global elements (settings, header, footer, etc.) once more. 

You must update the page to a paid version template if you have any pages using a free version template but want to use the paid version header, footer, or modules.