Spacing between the cogs on the cassette is different between an 8 and 9 speed derailleur, so the indexing in 9 speed shifters is different. If you look on eBay, you can probably find a Shimano cassette pretty cheaply and then you'd need a new chain to go with the shifters.
Really though, there's not a whole lot of benefit to going 9 speed if you have 8 speed. You'll get a slightly wider range of gear ratios between the two, but it won't be a huge upgrade.
Really though, there's not a whole lot of benefit to going 9 speed if you have 8 speed. You'll get a slightly wider range of gear ratios between the two, but it won't be a huge upgrade.