Top nuts are normally glued into place. I have never seen a interchangable top nut set but that doesn't mean they don't exist.
I have found that if the slots are carefully widened and the depth of the slots is not changed for a BEAD set up, that the extra .020" or so of width will make no difference when changed back to EADG.
If you are set on changing the top nut, it could be done by pinning the top nut instead of gluing it. This is a 5 string but will work for a 4.
I do this when I make top nuts from 3/16" or 1/4" thick brass or aluminum. The pins are small nails that are driven in, cut off and filed. The holes are .062" diameter. The pins keep the nut from moving side to side and the strings supply plenty of pressure to hold the nut tight to the neck.
mech