Customer placed resource upgrade order to buy 1 unit composite resource, which includes some physical resource in it.
After switching subscription to another Service Plan that also has some amount of the same physical resource included, it turned out that previously ordered amount of physical resource is not preserved.
Subscription has the resource limit according to the included amount of the new Service Plan only.
PBA-E called pem.upgradeSubscription with two parameters only : Service Template ID and Subscription ID, however there should be resource limit parameters as well.
The reason of the behavior is the deprecated POA API Protocol version 5 specified on PBA-E side:
Home > System > Settings > POA Integration
Change POA API Protocol version to 6