It would make no difference whatsoever. Due to the moment arm, a pilot carrying a laptop computer with him into the cockpit would probably have more effect than a passenger in the middle of the a/c moving one seat.
There are some VERY general guidelines for passenger loading. For example, perhaps 20 pax between rows 20-30, and 20 pax between 31-40. If he moved from row 29 to 30, it would be OK, but if he moved from row 30 to 31 it might exceed what's allowed on the paperwork. Perhaps that's what happened.
I'd hate to think that if a passenger got up in flight and went to the loo that I'd tip over! What a way to go!
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