The user_preferences.xml file contains all user-specific settings outside of the ones defined in a Project Configuration. These mainly relate to specific UI and performance settings.
The only relevant setting to change is the Configuration File that contains a list of projects. This can be done in a few ways, as listed below.
Alternatively, you can completely bypass modifying the User Preferences, and do a session-based override of the SBSCFG file by using a command-line argument on the Designer shortcut, see below.