If a client changes their market data status mid-month from Professional to Non-Professional the system will terminate any Professional market data and will look for the matching Non-Professional market data package. If there is an exact match, the system will automatically subscribe to the matching Non-Professional market data package and IBKR will issue a refund for the Professional market data subscription. The refund will be given typically on a one day lag from when the Non-Professional data was subscribed to. If there is no matching market data package, the Professional market data package will still be terminated but there will be no refund issued for that month.
If a client changes their market data status mid-month from Non-Professional to Professional, all of the Non-Professional services will be terminated, but any matching Professional services will not be automatically subscribed.