Hi.
The biggest risk you would have is with hardware no longer being supported. I am assuming everything is 64 bit already. I would also just make sure the firmware on your server is a recent version before you even look to upgrading. If I do recall there were some kernel changes we needed to make because some drivers had changed between versions, it was mainly to do with tapes drives. I just removed the offending drivers from the kernel and repeated the upgrade process.
Because of the library changes changed between versions I did find that some apps, Oracle especially, needed to be relinked to run properly.
I would also suggest that you load all the relevant CD's/DVD onto a depot on the server you are upgrading otherwise you could be in for a very long day/night. Add the latest quality pack to that depot too.
Lastly I would recommend that your boot disks are mirrored, split the mirror and use one of those disks as a backup. Also take an ignite backup of vg00 before you even attempt to do your upgrade. It does add a bit of time to the process, but in the end will save you a lot of time\pain if you need to roll back.
HTH
Andrew Y
Si hoc legere scis, nimis eruditionis habes