With regard to those services changing, it would appear that the seats are only changed when the train number is different to the one marked on your ticket...
In other words, if your train number is running, you keep your seats...If your original service has been merged into another train (different train number) and there is a different train number than that marked on your ticket, then you need new seats and ideally a change of tickets...
The contact centre seem to be having an 'It will be alright on the night' mentality, which from my recent experiences appears to have annoyed both staff and passengers alike, especially when travel becomes so important over christmas. The staff at the station are more proactive as they are seemingly trying to avoid these issues by sorting it out in advance of the departure.
On a separate note, I have seen some highly controversial changes to the Eurostar timetable, including the Disney service on Christmas Eve. It is imperative that anyone travelling that day directly to Disney is aware that it is leaving one hour earlier.
As for the train managers strike, well it was a flash in the pan and it appears to have been resolved last friday.
All the best.