The easiest way is probably really to boot with a MorphOS boot cd.
Afterwards change to the partition that you normally use when booting from hd. Here you should look for Prefs/env-archive/sys/PublicScreen.prefs and rename this file (it contains all the defined screen modes for the defined monitor).
Now, when booting from hd MorphOS should fall back to a "sane" screenmode (640x480 or so).
Selecting/defining a new monitor and new screenmodes should be no problem afterwards.
