I was wondering if it is possible to switch from a duel piston caliper to a single piston caliper.

Yes, all configurations will bolt up with the bracket and matching rotor.

you should be able to mount the single piston caliper to the existing caliper bracket and use the existing rotor, and use pads for the single piston.

Thank you for your replys..Can any one please tell me where I can get a repair manual for the 1999 Forester Subaru. I am finding it impossible to find one. With the exception of getting one at a dealer and that is way to expensive. Thank you in advance for your help.

Almost positive the rotors are different sizes. They were when I upgraded to dual piston calipers.