Context: We released a new Theme Library in October 2016. This is a collection of professionally designed page layouts that can be used as course landing pages. More info here
Installing one of the layouts from the Theme Library will not affect a previously active/customized theme. The new layout will be installed into the Course Landing Page Layouts folder of the active theme. So you can continue using the older layouts for some of your courses if you choose to.
That said, some of the snippets are different between the old and new layouts. For example, if you've customized the "course_landing_page_action_buttons" snippet, these changes will not be reflected in the new Mountain layout. This is because Mountain uses the snippet "mountain-1-6-1v1-0-0_course_landing_page_action_buttons" instead.
You'll be able to customize these new snippets in your theme once the layout has been installed.
@Jill Morton - This should answer your questions from this thread: http://help.thinkific.com/discussions/topics/313929. Just to be clear - this means that you will need to recustomize the "Free Trial" button if you switch to one of the new landing page layouts from the Theme Library.