Symptoms
Customer has a subscription based on Service Plan A. He purchased some additional amount of some resource and switched to Plan B, where this resource is also present.
It was expected, that purchased amount of resource is preserved after plan switch, according to the product documentation: If the new plan supports resources, purchased under the old plan in addition to included resources, they are preserved under the new plan.
However after plan switch, additional resource was removed.
Cause
Customer did the plan switch from the UX1. This issue has been confirmed by developers as internal issue OA-6560 (Additional Resource amount is silently removed from the subscription during Plan switch in UX1).
Resolution
As a workaround, provider can switch the plan from Provider's or Reseller's CP.