Implement subscription change

[pdobrescu]

Currently you can change a subscription by simply altering the plan attached to that subscription. The downside of this is that the entire history also gets changed.

An example where this model fails is:

• subscription A has plan B with the metered feature C, and 0 amount of metered feature included in plan B
• you change this plan for the same subscription A, to plan D, having the same metered feature associated, but having E metered feature included in plan D.

In this case, if the customer has consumed some metered features before the plan change, then there will be no charge for the amount E of metered feature, included in plan D.

I see two solutions for this:

Solution A

• When you change the plan for a subscription, it won't touch the previous plans included in that subscription. In other words preserve the plan history without altering it if the plan has changed. Right now when you change a plan, it will change it for all history.

Solution B

• When you change the plan for a subscription, end that subscription with the old plan at the end of the billing cycle and start a new subscription with the new plan starting with the next billing cycle.

Remains TBD which one is better.

[presslabs-bot]

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cc: @calind, @pdobrescu, @vtemian