Yeah, when I was having problems with my Cybervision board I just removed the Cybervision driver from DEVS:Monitors/ and then copied over the screenmode.prefs with one from a normal stock WB floppy disk.
It reverts to normal Hi-Res mode then. But then there is a major PROBLEM. The new Workbench 3.9 screenmode requester is TOO LARGE for the Hi-RES mode. The buttons for ACCEPT and CANCEL extend below the NTSC Hi-Res screen. You can see all your screenmodes listed, and can test them, but you cannot select them the with SAVE, USE and CANCEL buttons because they are cut off below the bottom edge of the screen! You you cannot then get out of Hi-Res mode into something more useable like 640x400 Interlace, etc.
In a way, this is sort of a bug/design flaw in OS3.9 (unless anyone knows a solution for this).