It should be about the same as doing a non bolstered western knife. I would remove the old handle, and then put the handle material up against the bolster, mark out, drill, cut out, and then reshape the handle/bolster at the same time. I have a Kikuichi Carbon Elite Gyuto I may be re handling in the next few days and it has a bolster. The only fly in the ointment is if the bolster and tang are not 90 degrees to each other with a squared off joint. I saw one knife with a bolster where the integral bolster was milled at a radius where it met the tang. I got my mom a Hiromoto AS a few years ago and I may re handle that one, too.
When I put together kit knives, I usually clamp both pieces of handle material together and drill thru them at the same time, using the blank as a template. For a bolstered knife, I would drill each side separately after making sure the bolster/handle had a nice meet up seam, and then epoxy it all at once. Sine the bolster is already shaped out, you can use that as a guide for doing the handle. If the handle material is fairly hard (stabilized wood or Micarta/G10), you can usually do the metal and wood together. Softer materials like bones and some fabricated stuff, I would mask off the bolster, shape the handle and polish it down to meet the bolster.
I a hoping that the handles are riveted on so I can drill the rivets and pop the old handles of fairly easily. If not, I will drag out the 2x72 belt sander I have a couple Tojiro's I may try my hand at as well. I just made up some of my own Denim Laminate like Micarta/JCarta that I may use on the Tojiro.
Hmmm. Japanese knives with Western style bolster or like Henkels, Wustoff, etc? I just knocked off the handle of a Tojiro Honesuki I bought years ago that has seen some abuse to completely refinish it. I was able to drill/Dremel off the rivet heads and knock them through with a punch and remove the handle. Not sure if the Western style knives like Henkels and Wustoff with the 3 rivets/pins showing are as easy or not. Some of those types of knives may have a molded handle; not sure how to remove it other than grinding it off and I don't know how long the tang would be.
Here is some of a tutorial for ya! I started to rehandle my Tojiro DP Honesuki because I am bored and I wanted to see what the red linen Micarta would look like on a knife!
I drilled out the rivet heads using a drill press and knocked the rivets out fully. The handles came off easily; there was no epoxy. I noticed that where the bolster met with the tang of the knife, there was a blob from welding the bolster on and the handle scales were relieved to go over this blob.
Next step was to cut an oversized handle scale from the Micarta using a band saw. I put the blank over the Micarta, traced around it and left it oversized, then cut it out on the band saw. From there, I took one handle scale at a time and my Dremel with a Carbide Burr and relieved the inside of the scale until it cleared the little blob and sat flatly on the tang. Once that's done, I checked how the front of the handle material fit with the back edge of the bolster. There was some light showing between the two, so I sanded a slight angle into the handle material to meet up better with the bolster. I repeated the same with the other handle scale.
Now it's time to drill. I used 1/8" SS pins, so I used a #30 drill bit (slightly oversized for the 1/8" pin material but it allows the pins to slide in easily and have room for epoxy). I made sure the handle slab was lined up and tight against the bolster area and carefully clamped it so that the scale wouldn't move. I then drilled my two holes. One thing to consider. When the holes in the tang are slightly oversized or are grooves instead of holes, I will often drill right at their edge. This helps when there isn't a bolster at the front because with the grooves, the handle can slide back and forth. By drilling at say the back edge of the front hole and the front edge of the back hole or vice versa, it helps lock the handle scale in place. After drilling one side, I put 2 drops of Super Glue on the handle and carefully glued that handle scale in place temporarily. I then took the other handle scale, put 2 drops of Super Glue on the scale and lined it up carefully with the bolster. Once the glue was dry, I used a clamp to hold all of the pieces together while I drilled the second handle scale, using the holes from the other scale as a template. (Normally I would just stack up both pieces of handle scale and clamp the blade on top and use the blade as my template and drill thru both at the same time). Once everything is drilled, out come the fitting pins, which are brass pins I use to hold the pieces of the handle together. I attached the handle scales together and put the blank on the top. Now I can trim very close to the blank on the bandsaw.
This is what it looks like all trimmed up pretty close:
Only 1 side with the tang up:
Here you can see the recess for the welded blob:
Once your handles are drilled and cut to close shape, I sand the back of the handles so they are roughed up for the epoxy and make sure everything is fitting nicely. I mix up the epoxy and start assembling the knife. I roll the pins in epoxy and push them from the bottom to the top of the handle material and then cover the whole scale with epoxy (Devcon 2 ton, JB Quik Weld, Rod Building epoxy like ProBond, U40, etc) and make sure to get the recess in the front as well as the whole front side of the handle scale that meets the bolster. Then I put the knife onto the handle slab and make sure everything is fit tight. I then coat the pins in epoxy again and coat either the tang of the knife and/or the handle slab, again getting the weld recess and front of the handle where it connects to the bolster and put the last scale onto the tang. I
It looks like the Western Knives with the 3 rivets should go the same as the Japanese ones with the Western Handles, but I dunno if they use epoxy or not. You may be able to boil the grips in water to get the epoxy to loosen possibly?