Imported from Forum:
How do you assign a global template to the preview? I cannot change the global template of the site as the new templates are still under development, and it needs to use the old template for publishing…However, I wanted to setup the preview with the new global template.
Will you please send screenshot of what you mean by that? Thanks! 
To test out the new Global Template for the whole web site, you can create a new Site Definition pointed at the same site folder as you current site, and set the Global template on that new Site to be the new Global Template.
All Page Templates that have their Global Template set to “Default” will pick up the Global Template from the Global Template on the Site definition in Publishing Design when previewing on that new site.
Note that the Global Template can also be set as a property of a folder. This lets you use different Global Templates for different sections of a web site so may be an alternative way to demonstrate the new design on some test content in a given folder.