First thing I would do is to make several outlines of the blade flat side on some fairly stiff paper for reference. You may want to cut one out later and hold it against the blade for reference.
Second: I would chuck it up in the WE and use the Sharpie trick to find out exactly what angles were used originally; including the location form the handle to the vise jaw, the angle cube readout on the flat side of the knife, angle cube reading on the sharp edge (you could use the cube on the bottom with one edge against the handle for a repeatable reference), as well as the angle cube reading on the diamond stone for the actual bevel. Write these parameters on one of your outlines sop that you can duplicate the settings easily. . You may have to move the knife back and forth and play with the tilt to get the best spot. If the sharpening angles of the stock knife change significantly along the blade, you may want to ask the owner to show you how it is used and determine if different angles are needed.
Third: This is the point of no return. Use an appropriate grit diamond (400 may be course enough, but you may have to ) and take off the all of the chips making a flat edge as others have done to de-stress the edge. (do a forum search on de-stress or destress for more information). Compare the new profile you have made to the original and do what you have to to make the curve smooth and approximate the original as well as possible. When you are satisfied that all of the chips are gone and the profile is acceptable, you want to remove any burr from the non beveled side of the knife and the new flat you have made so you wont fool yourself later into thinking you have a burr. You can do this on a bench strop or maybe a ceramic stone on the flat edge. I prefer a loaded bench strop. Then use a different colored sharpie on the new flat edge so you can see it clearly; you may have to re apply the sharpie several times along the way. I like to use red for the edge and blue or black for the bevels. The easy part is over.
Fourth: If you have a belt sander you can thin the fatter bevel a little at a time with power to save a bunch of time, but be careful not to go too far too fast. If you have a steady hand and can mount the knife securely in a flat position, a small angle grinder with sandpaper flap disk can do a lot of metal removal in a controlled manner; practice on something else, use really light pressure and wipe the blade with a wet rag on each pass if you try this; I use this on a lot of axes and splitting mauls. You can always put the knife back in the WE and sharpie and course diamond from time to time and judge your progress appropriately. Somebody with a lot of experience and the proper belt grinder / belt grit can whip this part out in minutes, but the rest of us just have to have patience and good judgement. Unless you are really experienced, stay away from the newly flattened edge with power grinding. If you don’t have power tools, you’ll just have to start with course diamonds and grind away; but along the way check the width of the bevel with a sharpie a number of times. You can make minor adjustments to the bevel by grinding proportionately more in the areas that need it, but you need to keep on track the whole way or you’ll end up with a wavy edge. Also check to see that the knife hasn’t shifted in the vise occasionally. If you have course diamond bench stones and want to save your WE stones, a lot of rough metal removal can be done by hand.
Fifth: When you have the largest bevel close to what you want, start working on the other bevels a little at a time. Work the grit progression on each bevel before moving to the next finer grit. You may have to go back to a courser grit sometimes but it is better than over grinding and putting too deep of scratches on any of the bevels. Be very careful of the final bevel and approach removing the sharpie mark on your newly made flat edge with no courser than 400 grit stones that are well broken in; you don’t want to have to start all over. This is really an experience judgement. You want to get a fine burr, but not chip the edge.
Sixth: follow the latest forum thinking and finish the edge with 1000 grit diamond lightly polished but not polished enough to remove the tooth form the 1000 grit. You may want to micro bevel the very edge at a slightly more obtuse angle than stock for less chipping. Lastly a couple strokes on a loaded bench strop on the flat side of the blade; don’t tilt the blade toward the edge.
Seventh: I hope you get lots of free sushi for all this effort. Keep the templates and angle dimensions for future touch ups and convince the owner that touching up the edge really removes less metal than a major sharpening.
I, for one, am a firm believer that removal of stressed metal at the edge of a quality knife is well worth the effort in the long run; this knife edge has obviously been stressed.
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