Lots written on this subject here, try searching too... In a nut shell, yes you can and it is a very common mod with excellent results for very little expense. I ran the 14/40 combo for a good 15K. I had no problems at all. I just replaced my chain and sprockets. I ended up reusing my 1000mile old 15t front along with a new chain and a new 43 tooth rear. I don't really see any difference other than it feels a lot smoother than the old set up. I would say that is a result of a new chain and sprockets more than anything else though. By the way, with 16K on the old chain and rear sprocket, the chain was absolutely spent. Lots of stiff links, and I would say most of the grease was long gone under the o-rings. The sprockets looked pretty good though. The stock steel rear and 14T front look like I could have used them again. I cleaned them up and put them on a shelf...