The cheeky answer is "with great difficulty". It's true though. Farmhouse sinks have an entirely different type of cabinet under them than a standard sink base. Granite cut outs are different from sink to sink as well (not just from undermount to farmhouse but from model to model). If I was doing it for myself, I'd pop the doors off the existing cabinet and remove the undermount sink from below, then tear out or modify the sides of the base cabinet as necessary. Finally, I'd call my slab sub to come out to cut and re-polish the granite, re-build the front of the base cabinet and slide the farmhouse sink in place. Then I would most likely tear out the granite top and replace it....because it would be an epic fail. I do this stuff for a living and the sheer number of variables, details and points at which it would all come apart are pretty daunting.