In your case, then, there is no problem at all. When you land in India with your new pistol, just sell off the old one and get the new one endorsed on your license.
Under the license regulations, you are given 3 months time after being issued a license to buy a gun of the de.scription entered in your license. Yes, under some circumstances the licensing authorities could extend this if you do not buy a gun. That said, I doubt that anyone would wait.