Hi Andrew,

You can use the Limit Plan app(http://take.ms/hDaS9) to automatically unpublish a plan when the subscribe limit reaches as configured in the plan.

But it is not possible to automatically publish another plan when this plan is unpublished, which means you have to publish the $30 plan manually.

Or you can also show both plans($20 and $30) in the subscription page, and indicate that the $20 is for the first 20 subscribers. And when the $20 plan is unpublished, users can only see the $30 plan.