How was the state of old compressor, did it frag and sent metal down the line? If you still have it around, try dumping out all the old oil. If it's close to black, chances are it's been grinding away and the iron particles have oxidized.
If there's moisture and gunk in the oil, it gets extremely thicker when it starts to freeze. It'll coat the walls of the evaporator and not transfer heat from the cabin. The refrigerant stays liquid until it reaches the outside line in the engine bay, then it can finally flash off and that's why frost builds up.