Rohit,Reducing the teeth's by two is always good for the chain and its revolutionary nature,however it also needs to be simultaneously compensated by the ration of the front sprocket as it is the one that runs along with it..
One sprocket reduction would do much harm and to the new chain set and the chain roller's would get cut or damaged soon..
I wud advise a complete change of the Chain Set along with front sprocket,rear and the chain.
Or do not do anything like what LP did with his Discover, he had changed only the rear to 42,it created a problem.
and the front 16-39 as rear, Again a problem..
Pls do not perform any such kind of stunts with BIke like he did..
You know him..
Don't You..
Now every thing is running fine with his De-sprockted 125CC, Have you ridden it..
i think when we were painting your rear rim