Thank you for your response.
After I stopped the Agent, I executed the alter statement and received following:
Msg 9776, Level 16, State 1, Line 1
Cannot enable the Service Broker in database "msdb" because the Service Broker GUID in the database (46BA6922-0A7F-40EF-8201-16F0961A7B6C) does not match the one in sys.databases (5D5541A9-C405-49A2-B01D-08D199ADB116).
Msg 5069, Level 16, State 1, Line 1
ALTER DATABASE statement failed.