Jan & John & Mike,
thanks for your responses. I will file an ticket to ask that these settings be deineable by user and not just by whatever the "application" thinks is appropriate.
Service Ticket number 8001542994 has been filed
After the reply from Bentley which confirms that the only defaults you can set the UPF back to are the "out of the box" Bentley defined settings, it appears that the best way to work with this is to make sure I have a good MS_USERPREFSEED file set up and just do the old delete UPF and let Microstation recreate a new UPF from that. This is fine - just needed to understand what that "Defaults" button was supposed to be used for!