I'm afraid it won't be that simple with BA. Your luggage will be checked through to BRU. BA will not interline on separate tickets, so you can't simply link your bookings and ask them to switch your destination.
Your options are:
- Fly to BRU as planned and make your way to AMS from there
- Fail to board the flight at LHR and ask for your luggage to be returned. This is a laborious process, so you'd need to allow plenty of time to collect and re-check.
- Get the itinerary repriced to route to AMS - this will probably be cheaper to do once you've commenced your journey because it'll be done at the historical fare. This will incur a change fee plus fare difference.