Yes, BA won't just do a change like this, it would only do it if you asked them to change it within the first 24 hours of purchases, otherwise they essentially rely on the fare rules. If you bought an inflexible ticket then making it flexible will come at a cost (where allowed). Now it is just possible that BRU will allow you to go on an earlier service, but the reality is that most BA services on a summer Friday out of BRU are likely to be full, so I certainly wouldn't count on it. By all means check in the Request Seat Availability thread (via the forum's Dashboard) a few days before travel to see whether the potential service is looking full or not. This can go both ways, if your booked flight is oversold it is possible the BRU agents will allow you to go earlier, they have been known to allow other changes there, though they probably aren't supposed to do it. Having said that, your connection should be OK, if you were fully mobile then a connection can be made at T5 with 45 minutes from arrival to departure, so in theory 90 minutes should be plenty. The other aspect is if either your BA flights have changed timing since you first booked, so have a look at the email you got with the e-ticket number on it, and compare with Manage My Booking now. If there has been a change of timings, even quite a small one, then you are allowed a free move.

But the best advice is if you like longer connections it is best to book them in the first place, or alter within 24 hours, thereafter is too late.

Your related thread is here:
https://www.flyertalk.com/forum/brit...ansfer-t5.html