Originally Posted by EJ123
So if I get a 9 speed chain and cassette with the LX shifter, it would all work?
Yes, of course.
The reason you can't mix and match is that the total cassette width is the same for both 8 and 9 speed systems. The difference is the spacing between the cogs is closer on the 9, so a correspondingly narrower chain is required. A 9 speed shifter will also pull less cable per shift to account for the tighter spacing.
Hope this helps.