>>47996119I know some GC Zelda game has a GBA minigame, but that used the Link Cable. I doubt that would be an issue tho since it might have just be a GC application to do the swapping from GC mode instead of from GBA mode, which just moved GB pokemon data form a certain box to the memory card and then moved that data from the MC to a GBA box, no need to have R/S running.
>>47996156I don't know every difference between G1/2 and G3 mons data wise, but can't the IVs be updated just by multiplying the old IV by 2 and randomly adding 0 or 1 to it? As an old IV of 4 can be 8 or 9. Then for abilities just give them one at random. All that matters its if the species, genre and moves are the same. EVs tho? Now that's a rabbit hole I don't want to get into since the GB method was 100% different. I guess just set them to 0 and make stats for Lvl100 mons still update somehow?