Vista - Upgrade Home Premium SP1 to Ultimate

I just got a new computer with Vista Home Premium preinstalled, and I need to
upgrade to Ultimate. When I try to upgrade the Upgrade option is disabled,
with a message "The upgrade cannot be started. To upgrade, cancel the
installation and then choose to upgrade to a version of Windows that is more
recent than the version you are currently running."

I assume this is because I have Premium SP1, and the Ultimate disc that I
just bought is the RTM version. Is there any way around this, or do I really
need to do a clean install and then update to SP1?

Thanks,
Steve Hoag