Q:

Can a left handed batsman change to right hand before the bowler starts bowling?

Field always set according to the batsman hand. Can a batsman change its hand before the bowler start bowling just to dodge the captain for setting the field?
Update: This question is about when caption is setting the field not when bowler is bowling

A:

A batsman can change from left- to right-handed whenever he likes. Law 36 section 3 says that his off-side is determined by his stance when the ball comes into play, which is when the bowler starts his runup.
You say you're asking about when the captain is "setting the field". Usually at this point the batsman is watching where the field is being placed, so wouldn't make a decision on stance until after this has taken place.

