In most countries (which have laws respecting copyright in the first place) it is illegal to download something that you don't specifically own, including variations on something you might own. If a game comes in two versions and you own one, you have the rights to that one version that you have and have no rights over any other version. In many countries you don't even have the right to download the version that you do own: in the USA, for example, even if you own a cartridge it would be illegal to download the ROM for that cartridge off the internet; you can use a ROM dumper to make your own copy, but downloading someone else's dumped copy is illegal.