nut replacement

If you have a JAN III brass for the same bass and overall nut width, but wanted to radically switch tunings (i.e. accomodate vastly different string gauges), could you do so by only switching the slotted upper portion of the JAN III with a new one having different slot widths? In other words...

I bought a Just a Nut replacement for my Dolphin SN 5-string bass and don't know what kind of glue to use. I watched the tutorial for replacing a nut but it didn't mention glue. The guy at guitar center said to use wood glue but it didn't match the glue on the tutorial and didn't seem right for...