The answer to your question depends on whether losing a significant amount of weight puts your body mass into the "average" range, or drops you into the "below average" category. Moving from an above average weight to something in the average range will allow you to request a reevaluation of your status and could result in lower premiums. Your original weight and your current weight will be compared to determine how the loss of weight has affected the risk you pose.
To determine your individual life insurance premiums, one of the factors used by insurance companies is the ratio of your weight to your height. Based on that ratio, insurance companies are able to determine your health risk based on an average build and proportion. If your ratio is too high or too low, your risk goes up, and your premiums increase accordingly.
In addition to lowering your rates by losing weight, you can also look for other discounts. Life insurance discounts are available for many reasons, including your age, occupation and where you live. Married people pay less for insurance than single ones, and people who do not smoke or drink are considered a lower risk. Ask your insurance company what discounts they offer and you may find that losing weight is only the tip of the iceberg for insurance savings.
Remember, all insurance companies are different, and the laws change from one state to another. The rules that would net you a savings in one state and with one company might be totally different with the same company in a different state. So losing a few pounds might earn a discount from one company but not from another. The best rule of thumb is to consult your insurance company and ask them what their procedure for dealing with weight loss might be.