I have an old giant warp D's 2 bike that's about 10 years old. I've maintained it and it still rides good and looks just as good. I wanted to upgrade from the old alivio 8 speed drivetrain to either a new alivio or altus 9 speed ddrivetrain but wasn't sure if the cassette would fit on the freehub on the bike and I didn't want to buy the components and then found out they couldn't fit and then be stuck with useless parts. Could somebody please help the old man stuck in a rut?
A 9 speed cassette will fit on an 8 speed hub. The extra gear is accommodated using thinner cogs and spacers. You will also need a new shifter as its the shifter that provides the indexing for the gears. If you also increase the range of gears (increase the largest cassette cog), you may need a new rear deraileur.
Its possible to run 8 speed chain rings with a 9 speed chain, but you will get better shifting upgrading the chain-rings. In this case, replacing the entire crank set might be cheaper.
There would be little advantage replacing the deraileurs (especially the front) unless they are worn out.