Once your 640x480 screenpack is active, you must configure your roster layout to match the new grid size. Open data/your_new_screenpack/select.def to add characters.
If you are mixing HD characters into your 640x480 build, utilize the localcoord parameter inside the individual character’s .def file. Setting localcoord = 320, 240 or 640, 480 forces the engine to scale high-res assets appropriately so they fit the standard-definition stages and screenpacks perfectly. mugen screenpack 640x480