My question is if I replace the old hub assembly with a new one meant for rear discs, can I just buy rotors, calipers and brake lines? Do I need to change the brake booster as well?

 

Since your car is an LX, I'm assuming it does not have ABS (was an option, but not many came with drums and ABS). When swapping to the rear disc, you'll just use your stock booster/master cylinder. The EX one will be different since its meant to work with ABS, which you probably don't have. You may want to switch out the proportioning valve out, but I'm not super familiar with the 6th Gen platform, so I couldn't tell you which would be the correct one. Changing the prop valve isn't a must, but it will help maximize breaking performance.