Also you can use epoxy to secure & fill in the gaps. Say if you have a black horn can buy black dye to add to the epoxy so it looks better with same color.
Thanks for your tips Keith. I might be able to practice the re-handling itself (not getting the handle ready for installation), because the Kato currently has a Ho-handle and I'm a lefty, so it needs to be flipped anyways.
Replacing Japanese style handles is not that hard. If getting a water buffalo ring handle you just need to make sure you have an old blade with a tang similar to your knife. Heat that up and burn in tang so your knife sits in the handle with about 2cm space from the top of the handle. Let it cool down fill with a little bit of epoxy or plastic glue, not full. Heat the tang up on your good knife just a bit, enough to melt the plastic/epoxy Insert it straight and once you can't push it in by hand anymore, use a mallet to hammer the bottom of the handle till the knife is hammered in to its desired spot.