So I decided to give the whole CARD1 to CARD2 conversion a go... Took the ROM, converted from card1 to card2, checked the header, it was still the same, used the card2 save tool to import the save file into the ROM, and booted it up. Online works so the header must be okay, but save file is still corrupt. I don't understand what's happening since it's the same ROM with the same, untouched header.