Change cog on Nexus 3 speed?
 

We were at the bike shop yesterday and my girlfriend tried out the Trek Belleville. She has fallen quit madly in love with the smooth, stable ride and comfortable upright position. (She mostly riddes road bikes recreationally.)

The only catch is that this bike is insanely heavy and geared for flat cities. We are looking at moving to Seattle which is fairly hilly. Obvious this steel beast will be no hill climber but is there any reason she couldn't switch the stock 19t cog to a larger (23t) one to lower the gear range a little bit? I don't think that it would hit the chainguard or anything.

It should be OK and lead to noticeably easier climbing. However, Sheldon advises that you might be better off using a cog with an even number of teeth, and my experience with switching cogs on my 3-speed confirms this. Details here:

http://www.sheldonbrown.com/chain-life.html

Cool. Thanks for the tip.