Having a plane go Tech is never easy for an airline to deal with.

In your example, I'm not sure that you can really expect the ground staff to start reaccommodating passengers onto the 286 until they know, for sure, that the 284 is not going to go. Even when you get the final word, re-booking the contents of one displaced 747 onto another is hardly as straightforward as you appear to think.

As adrianjc32 has already pointed out, there will sometimes be a need for a first-come-first-served allocation of the remaining seats on the other service, but even if all the displaced pax can be accommodated, the process cannot be done automatically. For example, unless pax travelling together are all on the same PNR, BA has no idea who is travelling with whom. This problem is exacerbated with the prevalence of OLCI.

Then there is the problem that most airports are singularly badly designed for ease of the re-booking process. The pax are airside, most of the facilities that would assist with re-booking are landside. This is likely to be a particularly acute problem at an airline's outstation rather than its home base or a hub.