Changing Subscription Pricing
If you offer all your content for free, and then switch to a subscription charge later, your existing students will continue to have access for free. New students will have to signup for the subscription before getting access.
If you change your existing subscription price, then current students will be grandfathered into the old price. New pricing will only affect those who haven't signed up yet.
If you ever decide that you want to raise the price for your existing subscribers as well, you can do this by editing the customer's subscription from within your Stripe account. Most creators will either want to let their current customers stay at the old price forever or will let them stay at the old price for a certain period, before changing their subscription plan.