If you start the demo mode there will be no changes to disk until you open the installer for both distros. Most distros will boot into the demo mode directly from the USB and then have a shortcut to start installing. Once you have created a bootable USB it will work with any device so you can test the distros out now with your current machined and when you get the new one you can just plug it in there and see if there are any hardware specific issues
I don’t know if this is what the authors meant, but YouTube has no access restrictions or DRM, so you can download the movies for ever