So I'm home at last. I surmise that the vol reprogramming offer was because they correctly foresaw that the plane would go inop and they were trying to reduce their EC261 liability.

Bonus: the flight to LHR was delayed too, due to a faulty engine deicing valve or something. It arrived a little less than 2 hours late, but I was just wondering: if it had arrived 2 hours and 1 minute late, would I have been eligible for compensation for that? I presume people just flying that leg would have been, but what about someone who (a) had that leg as the last leg of a > 3500 km total trip or (b) was already due compensation for the cancellation/delay to an earlier flight and so had been rebooked onto this new flight (I think there's case law but not 100% sure it applies here).