First of all, the new hip joint is likely to reduce your pain and improve your function. It's up to you to now increase your activity level.
If you can keep your intake of calories the same while increasing your activity, then you might be able to lose some weight. At the very least, you should work toward not gaining any more weight.
Studies do show that obese patients put increased loads on their joints. Grossly obese people may reduce their activity enough to balance out load on the joint.
A recent study was done looking at body mass index (BMI) and hip motion and function after hip replacement. Hip function was less as body weight increased but overall the change was minimal.