Just moved my flights too, LHR - AY over New Year. Very simple to do, now booked on AY flights whilst getting my BAEC avios bonus. Ironically, better flight times than I originally booked.
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Now I'm unable to select a seat on BA or Finnair (booking reference works on both). Any idea if seat selection will be available online shortly after the change (a processing time?) or should I phone BA/Finnair as instructed? I always find seat selection on the phone hard as I have no idea what's taken!