My seat (in Y) was changed between check in at HKG downtown & boarding less than 12 hrs later, and I doubt that this could have been due to a config change (unless they swapped the aircraft between the two LHR overnight flights). Guess it was needed for someone with special needs, family group etc.
I've had an upgrade once to WT+ at short notice in the other direction (early check in at Paddington, it was a few years back). I know that there was a plane change it may have been from a Low J to High J 747 which could explain it.
Seat changes happen.
Incidently with KLM you can (or could) go to any terminal at at Schiphol and change your seat which could be confusing if you don't throw the old BP away. I think that the same is true for Swiss.