I have an Islander MSC-4 and just got a wound Low-G string to experiment with but the string doesn't doesn't quite fit in the nut. Do I have to widen the nut to get that in there? If so, what does that do if I want to go back to a High G? Will it then work for both or will it be a Low G Uke from then on out?
I've been withholding my tuppence, waiting for someone with a bit more luthery experience to speak up, but no one has.
What I've done is widen the slot
only as much as I need in order to get the large string to seat properly. Widening it the least that I can get away with has allowed me to switch back to thinner strings without incident.
Now, while I don't need the string to be loose in the slot, I also don't want it pinching so tightly that I have to force it into place. (Some string materials don't mind being pinched a bit, while others will develop a weak spot there).
Sometimes I widen it toward the bass side or the treble side or both. For me, it depends largely on how I want the string spacing to end up on the particular uke in question.
If the string difference is so great that you will need a saw to widen the slot, it will probably end up too loose for switching back to a reentrant set. (It also might necessitate deepening the slot, which could also cause problems for switching back.)
Use thin nail files, or similar, and take your time. Also, be careful not to deepen it accidentally. If need be, file off the edge of your file to keep the teeth or abrasives from eating away at the bottom of the slot. And make sure your filing is level and parallel to the string direction (or, for some, the bottom of the slot angles slightly away from the string).
Always remember to smooth (and polish, if possible) the modified slot before using it.
Dan