How do I change a normal one-time order to a subscription order?

Due to technical limitations, you can't just turn on the auto-charging functionality for orders which weren't placed as a subscription. These orders don't have the credit card info and authorization linked to them. This is why you can't just turn on the auto-charging functionality for these orders.
If your customers want to have an auto-charging subscription, they will have to place a new auto-charging subscription order in your shop.

The other workaround here is to create a quick checkout link in Seal Subscriptions > Subscription rules > Quick Checkout Wizard and then send this link to the customer, so they can complete the initial subscription order and start a new auto-charging subscription.