Resizing VirtualBox disk

[david-8uUts6sDVDvs2Lz0fTdYFQ at public.gmane.org (David Kramer)]

Background: My last post about VirtualBox was about getting USB working,
and that was wildly successful.  I've been using it to sync my iPhone[0]
and VPN into work.

I need to make the Windows XP disk bigger.  Now that it's so useful I
want to install Office on it.  There are lots of tutorials out there,
but the newer and better ones refer to the fact that VBoxManage now has
a clonehd command to handle the messy part of copying over the contents
to the new larger VDI.
http://alittletothewright.com/index.php/2009/11/expanding-virtualbox-dynamic-vdis/

The problem I'm having is when I run that command, I get the following
confusing error message (indented for clarity):

david at lexa:~/.VirtualBox/VDI$ VBoxManage createhd
  --filename  WinXPBIG.vdi --size 20480
0%...10%...20%...30%...40%...50%...60%...70%...80%...90%...100%
Disk image created. UUID: 701d381e-0b13-4d8e-8b04-1ed2ff74983e

david at lexa:~/.VirtualBox/VDI$ VBoxManage clonehd
   --existing  WinXPPro.vdi WinXPBIG.vdi
VBoxManage: error: Cannot register the hard disk
'/media/extbara160/david/.VirtualBox/VDI/WinXPPro.vdi'
{f92bed62-be92-4322-b278-adda513d3f4b} because a hard disk
'/home/david/.VirtualBox/VDI/WinXPPro.vdi' with UUID
{f92bed62-be92-4322-b278-adda513d3f4b} already exists
VBoxManage: error: Details: code NS_ERROR_INVALID_ARG (0x80070057),
component VirtualBox, interface IVirtualBox, callee nsISupports
Context: "OpenMedium(Bstr(pszFilenameOrUuid).raw(), enmDevType,
AccessMode_ReadWrite, pMedium.asOutParam())" at line 209 of file
VBoxManageDisk.cpp

Obviously WinXPPro.vdi is the old 8GB image and WinXPBIG.vdi is the new
20GB image.

I have tried several variants on this command, and I can't figure out
what the problem is.  I did verify that
'f92bed62-be92-4322-b278-adda513d3f4b' is the UUID of the existing disk
by looking at the xml file for the VM.

What am I doing wrong?

Thanks in advance.

[0] which was a REALLY good thing, because earlier this week my iPhone
became one with the snow as I was running into my building at work.  I
now have an iPhone 4 and new 2 year contract.  Suckfully expensive, but
there you have it.