Let's see if this image works...
I've taken down and repaired a bunch of different lighters, but not an Old Boy of any make as of yet, so this is just going on what I have seen in front of me, and what I see in that pic. Based on the exploded layout above, it looks like you would (empty the gas), remove tamper, remove the fill valve cover, remove the center screw on the bottom, and possibly, hopefully, leave the flame adjustment regulator screw in place (hard to tell from the pic), then remove the bottom plate.
(IF the bottom plate hangs up on the flame adjustment screw and will not pass over it, you'll have to remove that too. Make sure gas is empty, and keep the BOTTOM of the lighter upright until you get the adjuster back in because you don't want to disturb the internals of the regulator assembly. If you have to take that screw out, take it out right before you pull the plate, and then put it right back in before proceeding with your work, in order to keep the regulator assembly intact and undisturbed. Also, adding gas while that adjustment screw is out, or removing that screw while the lighter has gas in it will most likely blow out the regulator pieces onto the floor or cause them to flip around in the regulator tube, and then you are possibly screwed.)
Once you get that bottom plate off, you will most likely be able to slide the sleeve that you want to replace off of the brass gas tank. My guess is that you would not have to take down any portion of the top part of the assembly, although you may have to slightly loosen the top plate in order to slide the sleeve off or to get the best fitment with the new one.
Good luck!
