I have just upgraded 8 to 9, if not enough space on root, system will offer to rearrange, this i don't like, it'll move partitions around and sizes may not be to your liking. If space OK, will use what's there and all should be OK. Make sure you get rid of old files, I left a large Patch file on root system and it caused me a headache with rearranging the Partitions.
Also you have to break mirrors if you have them, big risk is reattaching, on newer V200's, disks are a bit volatile and prone to errors. But you'd get this with an install as well. Either mthod has adavatages, upgrade saves you a load of work and overall quicker, Install is cleaner - all rubbish is removed. Depends how much time you have, if you upgrade and don't like it, Install!