Subscription resources can not be adjusted via BA CP: corresponding button is missing on the Subscription - Resources screen.
Trying to upgrade resources through the BA API gives this error:
Next Billing Date is in the past. Please wait while the subscription is billed and then try to upgrade the resources again.
This subscription's "NextBillingDate" is in the past, because the Account has not been billed since that date.
One of the reason is the subscription belonging to this account, that has Last Billing Date empty.
The field is empty because of internal issue PBA-64569 (APIs to create subscriptions do not validate StartDate, LastBillDate parameters) - subscription has been created by API with incorrect/missing StartDate, LastBillDate parameters.
Please contact Odin technical support for the solution.