after changing the screenmode, select USE. this will activate the screenmode, but won't save it, so you can get the old screenmode by rebooting. If you find the screenmode to be working, select SAVE. This will save the screenmode and activate it. When you restart, new screenmode will be used.
if something goes wrong, you need to delete prefs/env-archive/sys/screenmode.prefs by booting with no startup sequence.