Hmm, we can’t simply change the rule, because the preferred usage of en dash and em dash seems to be different depending on your language and country. In German we definitely want en dash when we write -- and I think it might be the same in French and British English, at least that’s my impression after skimming Dash - Wikipedia.
Would a site setting be enough? Should Discourse automatically switch between en dash and em dash based on the user’s interface language? I’m not sure what the best solution would be… 