Upgrading Sore 8 to a 9 should be OK.
New cassette, new chain and shifters. I believe the 'pull' of the cable is the same so 8-9 on a rear mech is OK. You may need a 1mm spacer, depending upon the wheels.
The only one that's 'different' was the 8 speed 7400 Dura Ace.....ahem....cough....so you can't go to 9 on that...