As Xulfiqar said, just transplanting drums instead of drums is not wise. One important thing to note is the braking force on both axles. If you change that, stability of the car under braking is compromised. The car may either fishtail, or lockup the front wheels and lose control.
The brake proportioning valve "divides" brake force between the front and rear brakes. Designing this valve takes into account the braking torque on both axles, the weight distribution of the car, the suspension movement (kinematics) under braking and expected load in the car (passengers, luggage etc.). Just changing from drums to discs may (not always) change the braking balance and you'll end up with even less braking performance than before.
I'd recondition the brakes to increase its performance to original, or you can also play around a bit with tires. If I had to change to 4 wheel discs, I'd transplant the whole braking system from another version of the same car - e.g. VTi braking system in an EXi etc. And in my opinion, it's a lot more trouble than its worth.
