Official comment

You can manually change the amount of the open order and all future charges will be at the new amount. Here is how to do it.
1, Open the Contact Record and then navigate to the Purchases tab.
2. Scroll down to the Subscriptions and Payment Plans section and click on the plan you want to change. Click Edit in the small Actions menu that appears.
3. On the Edit Open Order (status: current) pop up, edit the price you are charging this Contact. Then, click the Submit button:
This edit will affect this Contact's subscription only, and will not affect any other subscription.