Monitor change: Change frequencies at boot time



Hi,

My Debian displays on an old 15" Cathodic MaCom Monitor.
I plan to change it for a budget 19" Samtron.
Obviously I have to modify the Horizontal sync and Vertical refresh hz in
/etc/X11/XF86Config-4
When do I do that? Before or after changing the monitor?
How long do I have before it fries if the Hz aren't ok?

Hints and Advice wanted and welcome!

Jean-Benoit