Originally Posted by
brownlandshark
To convert from 9 speed to 10 speed you will need new shift levers, a chain, a rear cassette, a new 10 speed compatible inner chainring and a new rear deraiileur (because the pulleys are narrow for a 10 speed chain) You can get by with the 9 speed big ring and the 9 speed front derailler. Any 10 speed shimano cassette will fit on a 9 speed cassette body with the addition of the spacer which comes with the 10 speed cassettes for just that reason. No need for a new hub or wheel.
There's no such thing as a "10 speed compatible inner chainring." I can't comment on 5 speed chainrings as I have little experience with them, but everything I've seen used on 6, 7, 8, 9, and 10 speed are all the same thickness, ~1.9-2mm. The 10 speed rear deraillers use pulleys that are the same thickness as the chainrings which again means that there's nothing special about the rear deraillers for 10 speed.