I've upgraded from 7.1 to 8.4..but
I am assuming your probably going to have to do
7.1-8.4
8.4-8.5
8.5-9.0
might be able to skip the 8.5 step and go right from 8.4
Also- Since you're unix, what DB are you using. If DB2... Why switch? Also look into ICN.