Paid members after 30 days

Hello. I help run a hyperlocal community news site.
We want free open access without membership for any content published in the last 30 days (at any given time).
Any content older than 30 days will require paid membership.
That means, we do not want to offer free membership.
We don’t mind going back over all past posts (older than 30 days) to update their restriction to paid members only.
But, how do we offer the option of paid membership only?