Originally Posted by hueyhoolihan

if you have a derailleur bike that has been manufactured anytime in the past 20 years, yes you can, it's very easy.

remove the cassette and break the cogs loose or you can purchase new ones that you want. put it back together using extra spacers in place of the missing cogs. you may want to readjust your rear derailleur high and low limit screws too.

i've done this a number of times. going from 8 to 6 to 4 and eventually doing away with all but one (the simpliest solution of all). at which time one can remove the derailleur too.