|It looks as if you have a full version, upgrade disks are plainly labelled on the box with a red background and the disk itself is also clearly marked as an upgrade.|
It's usually the other way around, people wanting to clean install using an upgrade disk.
A quick Google finds that upgrading with a full disk can work, but there seems to be too many problems associated with it, (these all seem to be brand name pc's).
If there are must have extra's in Ultimate, then it may pay to clean install.
Hang on and see if anyone here has any experience doing it your way.