being mechanical engg............ i will clarify . sprocket is directly related to your top speed and torque.
when you increase the teeths of sprocket like 14 to 15 means you have increased the torque and decrease the speed . which means your engine will work under less load .
1) increasing no of teeths means high torque .... good in steep heights...... in Kashmir / northern areas
2) it may increase your fuel efficiency 3 to 6 km ....... depends how you drive .... if your RPM is b/w 3000 to 4000 all time
3) you can do one wheeling not recommended
4) increased pick
in the end i would say , alter means disturb .