Does an adapter for a flat mount caliper to post mount fork exist?
From bikehugger.com


  What about fitting flat-mount calipers to post-mount framesets? Think again. No such adapters exist, and if I were you I wouldn’t hold my breath waiting for them to appear. It’s probably not geometrically possible.


Sounds like from that site that it's pretty much not even possible to get it working. Best option might be to sell your old calipers and get some new ones or see if anybody in your area is looking for a trade.