Have you looked through the Knowledge Garden? There are a lot of articles listed which discuss the process, and where you must be in order to upgrade. You didn't mention how much customization (if any) was done, so even if there is a path from/to, it won't work. You also didn't mention what modules you have, the KG seems to indicate that the service release level required varies; same with the hardware/OS you are running it on.
Stacy