If you win your league's end of season playoff, you automatically promote to the next league level.
Also, current practice is to perform a bot cleanup after every season, moving bots down to the lowest levels possible. For example, in the US, division V is all bot, and division IV is partially bot & partially human. So, at the end of last season, any bots in league level I, II, or III were demoted until they reached level 4. Promotion during a bot cleanup is determined by regular season placement in your conference & record. I believe that all of the bot cleanup promotees in the US last season won their conference (but not the league playoff).