Tell BT you have broadband and make sure they confirm in writing that doing a renumber will not remove the service. When its done, tell PN of the new number so they update their records.
A renumber should not cause the broadband service to be removed but knowing BT they don't always get it right. Also confirm that if the broadband is ceased, then BT will pay for the reconnection charge that will be due. Again get this in writing!