Does editing a Sales Page Template automatically update all Sales Pages created with that template?

Page Templates are a source for creating pages but they are not master files. Changes made to a Template do not impact Sales Pages previously made with that Template.

No, Page Templates do not update Sales Pages. This goes for unpublished and published Sales Pages. 

Once a Sales Page is created through a Page Template it becomes it's own Page. The template no longer has any relationship to the Page. This is because Templates are not master files, like Assets are, rather Templates are a source file from which to start. 

Learn more about Page Templates

Page Templates are available with a SalesReach Pro subscription.