Upgrading from sarge to etch went fine. Vncserver is used for the X display as there is no monitor plugged into the server. X and vncserver remained fine after the install and worked ok.
For some X applications, Firefox, or Iceweasel as it was renamed in Debian, Font errors appeared. They were similar to:
Error: Cairo does not yet support the requested image format: Depth: 8 Alpha mask: 0x00000000 Red mask: 0x00000007 Green mask: 0x00000038 Blue mask: 0x000000c0 Please file an enhacement request (quoting the above) at: http://bugs.freedesktop.org/enter_bug.cgi?product=cairo xfce4-tips: /home/dajobe/dev/debian/cairo/cairo-1.2.4/src/cairo-image-surface.c:155: _cairo_format_from_pixman_format: Assertion `NOT_REACHED' failed.
After some googling I found that it was to do with the Display Depth (8bit) used by vncserver. The latest version of cairo does not support 8bit images. 16bit colour is required.
vncserver -geometry=“1024×768” -depth=“16”