The rear discs will swap, the proportioning vale on the firewall should work. Im not sure of the front brake lines. Remember if you swap discs you will have to swap the brake booster, master cylinder, Proportioning valve, and front brake lines.
I wonder if you need to swap the front gear as well. I believe the rotors are larger on the MX3 GS, and the calipers have a different part # but look the same. So, if you did replace the front brake lines, would that also mean replacing the rotors, calipers, etc??
This is assuming that the MX3 GS and Protege LX front assemblies are the same.