I just got a nice big new monitor

The command is

dpkg-reconfigure xserver-xfree86

But this will silently fail to update the XF86Config-4 file if the
file has been modified, if you read the file carefully you will see a
useful explanation - but I missed this.

# XF86Config-4 (XFree86 X Window System server configuration file)
#
# This file was generated by dexconf, the Debian X Configuration tool, using
# values from the debconf database.
#
# Edit this file with caution, and see the XF86Config-4 manual page.
# (Type "man XF86Config-4" at the shell prompt.)
#
# This file is automatically updated on xserver-xfree86 package upgrades *only*
# if it has not been modified since the last upgrade of the xserver-xfree86
# package.
#
# If you have edited this file but would like it to be automatically updated
# again, run the following commands as root:
#
#   cp /etc/X11/XF86Config-4 /etc/X11/XF86Config-4.custom
#   md5sum /etc/X11/XF86Config-4 >/var/lib/xfree86/XF86Config-4.md5sum
#   dpkg-reconfigure xserver-xfree86

see also
www.tulrich.com/geekstuff/debian_tricks.html
bugs.debian.org/cgi-bin/bugreport.cgi?bug=223929