My outbound has now flipped to LHR, with a change of flight number (albeit still in the LGW 4 digit range) with only a marginal change in timings. The messaging is a bit crap though:
* app just shows the original flight as cancelled
* an email announcing a seat change (even though they have preserved the seat numbers i was allocated
* MMB correctly shows the cancellation and rebooking
quite pleased with the shift to LHR - First Wing and a coffee in GF (even if it will be full with Silvers “maximising” the experience 😉