In the event, you have a customer who subscribed to one plan and then wants to change their subscription plan to, for example, go on a more expensive or less expensive plan, you can easily change their active plan and their monthly charge. 

If you visit the subscription detail for a customer's subscription that is subscribed to a plan based project, you'll find a Change Plan button.

When clicking on it a modal window will open where you can select a new plan for the subscription.

When a new plan is selected the change in the monthly plan amount will be shown with the difference between the current and new situation.

The new plan price will be charged on the next regular scheduled billing moment.

So if a plan changes from €10/month to €20/month five days in advance of your regular billing moment, the regular billing moment will become €20.

Informing the customer about a plan change
There is no automated email yet that informs the customer about a change in plan. Make sure to contact the customer manually and confirm the change to their subscription.