How to remove a drop-in porcelain sink under a countertop?

My porcelain sink has a few chips and my wife wants it replaced. It looks like an under-mount but when I look from under, it sits on top of the plywood. So, it is a drop-in (on the plywood) before they put the countertop on. I don't want to remove the countertop as it is a BIG piece. Maybe I can use a router or some sort of cutter to cut around (see the red line in the last picture) and drop it down to the bottom to remove it, then install a drop-in? How do you cut a porcelain sink? Router with a special bit? Angled cutter or some sort? Or hammer? Attachment 43994 (http://www.doityourself.com/forum/attachments/toilets-sinks-showers-dishwashers-tubs-garbage-disposals/43994-how-remove-drop-porcelain-sink-under-countertop-img_3186.jpg) Attachment 43995 (http://www.doityourself.com/forum/attachments/toilets-sinks-showers-dishwashers-tubs-garbage-disposals/43995-how-remove-drop-porcelain-sink-under-countertop-img_3187.jpg)