When trying to upgrade subscription in the CBM by adding add-on in the following way:
Go to CBM -> Subscriptions -> Upgrade and select in item "When should this upgrade take place" - Upgrade now and DO NOT apply proration credits. Select Add-on and click Adjust Add-on.
The new invoice is not generated.
It is not possible to upgrade or downgrade this subscription again till the end of the current billing cycle.
According to the next guide, Upgrading and Downgrading Subscriptions:
Upgrade/Downgrade now and DO NOT apply proration credits. If it is selected, the system performs the following:
Creates the event to provision the upgraded or downgraded subscription to the Panel. The next invoice (for the new plan) will be generated as usual before the next billing cycle. Option Generate invoice immediately is not taken into account in this case.