The plan id associated with the API subscription. This value is equal to the id field of the Subscription Plans resource.
A plan can be upgraded at any time. But once upgraded, the plan can not be downgraded, and the account can not be cancelled, until the first day of the next billing cycle.
Free accounts can not be upgraded. A new account must be created.
A plan can be downgraded at any time, provided the plan has not been previously upgraded during the same billing cycle.