Add functionality for Upgrade / Downgrade

Problem/Motivation

See

• https://developer.billwerk.io/docs/subscriptionJS/portal
• https://developer.billwerk.io/docs/useCases/contracts/upDowngradingToPlan

Payment For Up-/Downgrades

For security reasons, you can't actually perform the up/downgrade solely through the frontend. Instead, you'll have to create an order from your backend using the REST API. Please read here.

You'll want to ensure that up- or downgrades requested by the user are actually valid. For instance, a downgrade might not be allowed at this time or in conjunction with a given component that the customer is subscribed to. Since this type of business logic can generally be very complex, it makes sense to have the implementation on your side.

To pay for an up- or downgrade you are able to use two workflows:

Paying for an up-/downgrade synchronously
Paying for an up-/downgrade interactively

https://swagger.billwerk.io/#/operations/Orders/post_api_v1_orders

Steps to reproduce

Proposed resolution

Remaining tasks

User interface changes

API changes

Data model changes