It’s important to drain and replace the brake fluid from the master cylinder, brake lines, calipers and wheel cylinders, because not only to air bubbles get trapped in the lines, but because moister seeps into the system. As the brake fluid absorbs water, its boiling temperature lowers, which can lead to some sever problems down the road. Brake fluid should be replaced every two years.
Is it okay to replace the front brakes and not the back ones? Sure, if your rear brakes are still in good condition. Your front brakes often wear out before the rear ones do because they handle a higher percentage of the breaking load.