Next time try calling the ARO office, instead of the ground controller.
Can I modify (delay, cancel or move forward) a FPL that I have submitted via the Belgocontrol website ?
A: For the moment it is not yet possible for a user to make any change to his/her FPLs via the website. Any change should be made by calling the Belgocontrol ARO office (+32 2 206 25 40). The tabsheet ‘FPL Related Messages' is only intended for ARO staff and aeronautical authorities.