I recently went to visit Brent Auger at DRAGONtail Tenkara to buy a Mizuchi tenkara rod as a Christmas gift for a friend. In talking with Brent, he stated that the Mizuchi's handle has been updated and refined to more look like the handle that comes on the DRAGONtail Mutant.
The original Mizuchi handle is larger in diameter and has a less dramatic gourd or camel shape. Because of this, some people felt that the handle was too large for their palms to be comfortable. I don't find this to be true for me, but I wear size XL gloves and find the larger handle to be less fatiguing to my hand during fishing.
So to accommodate folks with smaller hands, the handle has been refined. It is less massive overall, has a more noticeable gourd or camel shape, and has a smaller diameter. I think it looks really nice!
If you have a Mizuchi, and think you might prefer the new handle shape, contact Brent. I don't know cost or availability, but I'm sure he'll be able to help you!