Perfectly safe to do if you have a cloned backup of your system drive under 10.4.
If we say “clone the system drive” enough then it will hopefully stop some poor person upgrading without backup and losing weeks or months of work!
My upgrade with BMD card and Sonnet SATA RAID card has gone OK so far – although I am expecting trouble ahead, but I’m not worried because I’ve cloned my system drive 🙂