The SEC isn't a lawmaking body; its job is to decide whether somebody is breaking the law as it stands. So there is no question of "before it was illegal" or ex

The SEC isn't a lawmaking body; its job is to decide whether somebody is breaking the law as it stands. So there is no question of "before it was illegal" or ex post facto law here: the question is whether cryptocurrencies (or some subset of them that meet specific criteria) are breaking existing law. If the SEC takes this case (or any of several similar ones) to court and wins, that doesn't mean the cryptocurrencies involved have suddenly become illegal: it means they were illegal all along, it just took law enforcement a while to catch them.