Thanks.
I looked in their docs.
It seems, this being one of those flash types that can't be overwritten by simple upgrade, that this is not trivial (see http://wiki.openwrt.org/doc/howto/generic.flashing - see the note "Sometimes the original or factory firmware will only allow you to flash your device with a specific firmware file. If that is the case, you will not be able to install OpenWrt using this method." - you have to use the complicated method listed there).
I'll think about it, but I'll probably simply buy a new AP.
Thanks
NK