Before I Use the Primary for a Template...

Looking for a little more clarity please...on the Settings>Blog Templates page, it says "...this will turn the blog you selected into a template blog". (There is only one 'blog' installed as the primary site, and only one theme for it.) I want it to remain the primary, and I want to create a template that is a copy/clone of it to use as the master template for all future sites.

If I use the primary to create a new blog template (to be the master template), does the process make any changes to the primary, or does the above statement in quotes mean that the new blog template is a COPY of the primary and is turned into a template and no changes to the primary...?

Following the prior quote is "Any changes you make to this blog will change the template"...which sounds like the primary IS the new template...this is desirable in my case...I want changes made to the master template to change any sites thereafter so all sites get any recent changes...

It seems to make more sense based on the instructions to 'create a new blog first', but that would mean customizing the theme again completely - a huge job...apologies if this is basic, but a little paranoid about leaping before looking...thanks in advance.