Re: switching Bouzouki strings from octave to unison
I'd say whether or not you add tension, the chances are good that the neck will bow eventually. I question the judgement of a builder who makes a very long necked instrument without a truss rod or at least some other reinforcement. If this change is important to you, I'd suggest stringing relatively lightly for the scale just to be on the safe side.
It is not hard to widen the nut and bridge slots. You will need files that are the correct size for the gauge strings that you are putting on. You also have to be very careful as you are filing not to let the slot "creep" over towards the string it's paired with. If the string spacing becomes too close, the strings will hit each other when played, creating a buzz you will probably not enjoy. If anything, let the slot expand AWAY from the paired string.
So it's not hard. But it is delicate work. VERY tiny increments of slot depth, width and placement make a big difference in playability and tone. Some experience with the materials and files makes the difference between whether it is done properly or not. If you have an experienced luthier (who already has the files on hand) in your area, it might be worth getting them to do it rather than purchasing files (that you will not likely use much afterwards) and risking messing up your nut or bridge. If you are keen to do it yourself, proceed carefully. If you can practice with the files on some similar material first (to get a feel for how it moves, cuts, etc.) it would be a good idea.
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