Can the rear drumbrakes be changed to disc?

Sure, but it's not practical. Drum brakes are already real effective at making the tires skid. What more do you want them to do?

You have to consider that disc brake calipers take a lot more foot pressure to operate. The master cylinder will be different. Drum brakes have a residual check valve in the port to maintain 10 pounds of pressure on the brake fluid. You can't have that valve with disc brakes or they'll stay applied while you're driving. Most cars with anti-lock brakes now come with disc brakes in the rear. If you have ABS, the hydraulic controller is designed and calibrated for that system. If you don't have ABS, you'll need to change the combination valve. It includes a proportioning valve that limits rear brake fluid pressure to prevent easy rear-wheel lockup.

I watched a friend convert his Dodge Neon to rear disc brakes, and when he got done, he couldn't tell the difference.