The entire fleet has to receive upgraded engines and so it's possible that some of the aircraft that haven't been grounded for a long period could have had them fitted already. They'd only have to be stopped for a couple of days for the engines to be changed and several have been in and out of maintenance at LHR since the global engine rollout started in earnest so it's at least theoretically possible. Unless someone with inside knowledge gives us a rollout progress report there is no way to tell how many have been done and which was the first.