Ok, after a couple of brain farts shook loose my CRS I found the answer and
will share for those who may do the same thing.

sudo cp /etc/X11/xorg.conf /etc/X11/xorg.conf_backup : Make Backup File
sudo /etc/init.d/gdm stop (or kdm for KDE) : Stop X
sudo dpkg-reconfigure xserver-xorg : Setup X
sudo /etc/init.d/gdm start (or kdm for KDE) :Restart X

Bit shorter - for stopping the xserver, sudo killall gdm (or xdm, or
kdm, whatever) works with no apparent ill effects. To restart it I just
type sudo gdm (or xdm, or kdm, or whatever).

Another note - if you ever switch which PCI/AGP slot your video card
goes in, you'll probably need to run the command, unless they've fixed
it somewhere along the line.

I doubt that there really are ill effects, but myself, I think using the
init.d scripts for shutting down services are preferable to using
killall. Killall is like using a shotgun to turn off the lights rather
than just hitting the switch.

While we are talking about video problems after a HD swap...

I have 2 computers with the HDs in pull-out trays. I installed 7.10 onto
the one with an AGP display interface.

Putting the HD into another computer, a Dell Inspiration 4400 with a PCI
interface, the video is way out of sync.

This is probably because the computer Ubuntu was installed on a computer
with a 22" wide-screen 1680 x 1050 LCD and the Dell has a 19" standard LCD.

Is there a simple cure for this? May be hook the wide-screen to the Dell,
make the video adaptor corrections to say 1024 x 768 and then hook-up the
19".

Is there a way to get Ubuntu to back-down on the resolution while it is
booting up? Or may be do it after booting up by and a few keyboard strokes
that does not require seeing what is on the monitor.

Isn't there a way to scroll thru the video modes, something like ctl alt + ??

Now and then, I'll inadvertently boot my Ubuntu 7.10 (on a Dell
Dimension 4400) with the A/B KVM (keyboard-video-mouse) switch set to
the other computer. Ubuntu comes up in a "lowest common denominator"
resolution when that happens - 800 x 600, I think.

Might be worth trying to boot with the monitor powered down. Easy
enough to try.