Great.
It worked a treat.
Actually, I found an easier way to do this...
I tried 'apt-get install sysvinit/unstable' to force the switch. Essentially this should work, but dpkg falls over on sysv-rc - I think this is due to change in dependencies between the versions of sysvinit in woody and sid....
I updated my /etc/apt/sources.list to point to stable and testing as well and tried 'apt-get install sysvinit/testing' getting the same problem and then 'apt-get install sysinit/stable' which solved the problem. This also fixed up all the dependencies so I go then go ahead and upgrade to the unstable version.
Having done this, I was able to 'apt-get --purge remove knoppix*' to remove all the neat little programs like 'soundcardconfig-knoppix' that I no longer need.