NPC Code:
removetiledefs;
addtiledef <image>,<levelnamestart>,<type>;
put that in a npc in the level or a weapon.
image = the image of the new tileset
levelnamestart = the name/part of the name of the level you wish to use the tileset with
type = Two most common types for setting which tile does what are 0 (default tileset) or 1 for the common new tileset (probably the one you want)