Downgrade / Upgrade existing subscriptions

[pozylon]

Currently, as long as a subscription is INITIAL, updatePlan will work accordingly but it fails when it's not INITIAL. It should be possible to change the product in some cases, for example you have:

• Product A Monthly
• Product A Yearly

or

• Product A 2 times a week
• Product A 4 times a week

You should be able to upgrade an existing subscription from monthly to yearly or maybe any other valid business case. Business logic has to be written to do a pro-rata credit note or equivalent to stop the former subscription pre-termination.

[pozylon]

This has been touched here: #278

[pozylon]

Postponed until we have a real customer project where we need all that subscription logic