The ITK come off easily. Other brands I have tried and found that they use a flexible sealant that will not budge with force or heat; I have had to crack a few handles in half to remove them when they are sealed like that!!
I do my rehandles a tad different
One way I do it is to use 1/2" OD, 3/8" ID aluminum tubing. I drill a 1/2" hole 2" down into the rear handle part and 3/4 the way thru the ferrule. I then grind the tang down to fit into the aluminum tubing and then file/Dremel the slot on the ferrule for the tang to slip into. Once everything is fit well, I epoxy the handle pieces to the aluminum tubing, which also helps to strengthen the joint between the ferrule/rear grip portion. I shape out and finish the handle and then install the handle onto the tang with epoxy.
Second way is to use a 1/2" wooden dowel in place of the aluminum tubing; the dowels are somewhat soft and will be easier to file the slot than stabilized burl woods. You can also use a saw and slit the dowel for the tang, but you have to work quick; I've slit the dowels and the next day, the ends curled in towards each other and I barely got the tang into the slot. You can also put the tang into the dowel slot to keep it from moving as much and to keep it open with the tang itself or a wedge. You can either do the dowel all the way thru the ferrule and partway into the rear grip, or 3/4 thru the ferrule and partway into the rear grip and then file the slot in the ferrule. Once the handle is fitted to the tang, I finish out the handle (Octagon, D or Oval, or Western style handle) completely and when it's all done, buffed and polished and coated with a few coats of finish, I epoxy it to the tang.
If you want a 1 piece handle, get a set of scales or split a block in half, carve out a cavity on both sides for the tang and then epoxy it all back together, shape out and then install to the tang.