Even with mass-energy equivalency, it is much easier to change mass into energy than the other way around. But as far as changing an object's *weight* by pressing a button, that should be possible. Weight is dependent on both on the mass and distance separating two (or more) objects, or the acceleration of an object. If pressing the button initiates something to accelerate that object away from another object then its weight will change...

(Note: Pedantic me has not yet watched Eureka, so I don't know how this weight-changing object is used)