When your map is loaded, go to... Advanced->Modify Tileset (select base tileset)
When you select another tileset it will automatically replace any tiles you already have on the map with the tiles in the same positions on the new tileset.
Ghan has said he has fixed this. Monovertex please confirm this fix. This was only a problem with people that had signatures in the upper levels like not the special members but the respected members.