What if I choose replacement tires narrower than my current tires?
2.8 is bigger than 2.10. 

When you run a bigger tire, you have to worry about the tire rubbing on the frame or fork.

Running smaller tires are usually simpler.

Tire sizes have to be appropriate for the rim width; wide tire on narrow rim could have some control issues/sloppiness (and if you had rim brakes, that could also cause some rubbing). Narrow tire on wide rim could increase risk of damage from hazards. See this page for details: http://www.sheldonbrown.com/tire-sizing.html