For longer-time users who are still on the non-premium account there are premium features they have never experienced as new users do when they sign up today. Maybe it’s time to randomly dole out some free 7-day premium upgrades to let those old-timers experience the new stuff and perhaps convince them to upgrade permanently?
I know from own experience there’s a month free extension for those who try to cancel their subscription, so this could be another marketing tactic in micro∙blog’s arsenal.