There are many ways to switch x org configs.
Typically this is not needed anymore because
of auto-detection + xrandr.

In rare cases you still need manual intervention:
for example if the monitor is not directly
connected with the graphics card and may not
be changed dynamically.

== as selection item in grub ==

install grub2-splashimages (I did not find out how to use gfxboot
because of lack of docu)

modify the TIMEOUT in /etc/default/grub
so that the user gets a good chance to select something

use a nice wallpaper by putting
at the begin of /etc/grub.d/05_debian_theme

and run
 sudo update-grub

modify linux_entry and add following entries to /etc/grub.d/10_linux
see the file in http://www.markus-raab.org/Info/Config/grub_10_linux

and write following script to read the cmdline during bootup and configure your system accordingly:
see the file in http://www.markus-raab.org/Info/Config/switch%2dxconfig

== as selection item in DM ==

(NOTE: Did not work and I gave it up in favor of grub method)

create the file:

1 [Desktop Entry]
2 Encoding=UTF-8
3 Type=XSession
4 Exec=sudo /usr/local/bin/switch-beamer
5 TryExec=sudo /usr/local/bin/switch-beamer
6 Name=beamer
7 Comment=Switch to beamer resolution
8 Comment[de]=Auf Beamer Auflösung umstellen

and the file /usr/local/bin/switch-beamer

1 rm /etc/X11/xorg.conf
2 ln -s /etc/X11/xorg.conf-beamer /etc/X11/xorg.conf
4 PID=`ps axo pid,args | grep X | grep $DISPLAY | awk '{print $1}'`
5 kill $PID

Fr Jul 30 13:54:24 CEST 2021
patent_button.gif valid-html401.png elektra.jpg fsfe-logo.png valid-css.png vim.gif anybrowser.gif