Apparently, it was not just for flow rate reasons for the switch. They had a fire engine water arch thing to welcome the aircraft.
I do wish they had told me; it was depressing waiting in the wilds of Hounslow for the arrival, only to watch it go behind me!
A couple of hours later, they pulled the same stunt. Having moved to the 27L approach, I was looking forward to seeing the inbound LOT B787. And I did indeed see it. A mile away on 27R!