mtb addict is the only one to mention the changes in geometry. Adding drop bars stretches the riding position, so while the rider might be very comfortable with the geo and is merely seeking hand position variety, adding the drops might very well stretch the rider uncomfortably.

Frame size recommendations are different depending on flat bar vs drop bar configurations. One simply can't just throw on a set of drop bars and expect the same reach. Riding on the hoods (or drops) is several cm's ahead of the end of the stem. And before someone mentions swapping to a shorter stem, consider the handling compromise when the stem is shortened by those same several cm's.

Of course, if the frame was too small to begin with then go for it.

-Kedosto