How do I change the starting file number on a D600.

There's a pretty easy way to do that. Let's say your pics are named with the format IMGxxxx.nef. And you want your first picture to be IMG1234.nef. Take one picture, get the SD in your computer and rename it to IMG1233.nef. Next in your camera go in custom setting d7 "File number sequence", choose reset, then put the setting back to on. Put the SD card back in your camera, and the camera will start the sequence from the highest picture it finds, so the next picture you'll take will be IMG1234.nef.