We're working on the upgrade to SP 30 right now and doing a small rollout of Client 2008 whilst we're at it. I'm bringing this up because it wasn't clear to me when I tried it, so I apologize if this seems overly simple. When you run the setup and get all of HP's pretty GUI stuff, the "Upgrade client" option only seems to upgrade the "old" client. You have to drill through the folder structure to find the Client2008.msp to update it. Not sure why they didn't add it to the fancy update screens.
Ha! That's too funny. It never occurred to me to run the autorun.html instead of the setup.exe from the extracted folder. That should teach me to use someone else's download/extracted files rather than going through it all from download to finish. The autorun.html gives the option for all the installs. Setup.exe skips Client 2008. Good to know for future reference.