14 DAY TRIAL // The only reason that comes to my mind for a user to want to change the default web browser on a frequent basis is for testing purposes. Well, if you want to do this without starting the surfing tool, there is an app that does it.
Rather than addressing the issue each time from the configuration panel of the desired application, there is one simple solution that enables you to do just that, without launching a web browser: Change Default Browser.
This is a simple tool but you have to go through the proper installation steps in order to use it on the system. Hopefully the developer will come up with a portable version as well.
It supports all major browsers (Internet Explorer, Mozilla Firefox, Google Chrome and Opera) and detects the installation path for their executable automatically. If the location is not the default one you can punch it in from the application window.