You can extract the graphics with
so all GFX files show up in the Graphics folder, assuming the joined GFX files option isn't ticked. Since you want the post-special world sprites to replace the regular sprites, you must open GFX31 with YY-CHR
, copy them and paste them in GFX01 to overwrite the existing enemy sprites.
Another option is to open the overworld editor and go to Overworld > Change Special World Levels. When in there, change one of the levels to the one you start. Since you want to swap the sprite graphics, that's the very last level.