Yes, it is risky. You need to find an employer in India transfer your visa, then move to USA, where you have an actual job to join on arrival.
Using previous employer's letter and travelling without their knowledge would put you under the risk of deportation and getting banned from entering the country in future. The immigration officials may start questioning you based on documents and may dial up your previous employer for details.
Always do what is right, don't do anything illegal that may cause you trouble in future.