If you are simply missing some single sprocket you'd like in an 8 speed cassette, for instance, you could take out one that you don't need and replace the empty with what you'd like. (If you replace your top gear though, you couldn't just move the next up but would have to replace the first one with a dedicated first position sprocket.)
Depending on what you're missing in your drivetrain that might be better for some people than sinking money into a lot of upgrading. Personally I miss about mid 50s gear inches for grades or just lollygagging and that's in a 16 tooth for a lot of compacts. Not too many 8 speed cassettes with the somewhat larger climbing end-sprocket give you a 16 along with it unless you modify it.
Last edited by Zinger; 09-23-13 at 05:16 AM.