To convert rear drum to disc you will need an aftermarket proportioning valve and master cylinder. Stock disk/drum masters do not provided the fluid volume needed for disc. To this point, you will notice that the bowl on the cylinder is bigger for the disc and smaller for the rear drum. They have to be the same for disc/disc. Good luck.
The correct answer to your question is "your mileage may vary!"
Your existing prop valve is a good place to start!
The rest depends on your car.....wheels and tires (contact patch), rake on suspension, rear rotor size.........
You generally do not need to replace your M/C! Start with what you have! The smaller reservoir is just that, a smaller reservoir! Just means as your pads wear and the pistons move inwards drawing brake fluid from the reservoir.....that you may need to add some more fluid from time to time!
Your REAL concern is if the rear circuit of the M/C has a sort of back flow valve in it....to prevent the drum brake shoes from slamming back when you take your foot off the brake pedal. Disc brakes don't work that way.....and the valves generally tend to not be all that restrictive for small movements/volume as you would have with disc calipers vs drum wheel cylinders!
If you find that your rear wheels are staying activated, then you need to look more closely at your rear M/C circuit. Any inline valves that may have been installed, to keep pressure in the lines, WOULD need to be removed. But, I do not recall any "factory" use of these valves.........would be something that a well meaning mechanic installed.