I strung up one of my ukes low G, and even though the low g string didn't fit into the nut, it still sounded good. I didn't want to alter the nut, because I wanted the option to go back to high G if I wanted. If you have a removable nut (some of them are loose or just lightly glued) you could get a second nut made for the low g, and keep the original nut unchanged. In my case, the nut was not removable, so I just let the string ride on top (it didn't seem to hurt the intonation in this case).
–Lori