If people sign up for your free trial but don’t upgrade, you can automatically downgrade them to a forever free plan with a limited features set, and keep nurturing them.

Offering a limited features set for free allows you to stay in touch with users that haven't converted.

The goal is to keep your users accustomed to using your product in order to create new conversion and upsell opportunities later on.

Keep in mind that free users can become the biggest advocates for your product and contribute to acquiring paying customers, sometimes even more than paying users.