Originally Posted by dmurphynj

Not only possible, but very likely.

That flight is operated by a Boeing 787-9 aircraft. Maximum range on a 787-9 is 7,565 nautical miles.
JNB-EWR is 6,943 nautical miles.

Pretty close to the maximum distance the plane can fly. So weight-and-balance calculations are pretty dang important - especially when that flight is going to be loaded with cargo.

There simply isn't much "fluff" in the numbers.

I'm sure not getting the upgrade hurts, but - especially for takeoff and landing - the pilots have to do a bunch of math to calculate the proper center of gravity, thrust required, takeoff rollout distance, landing runway length needed in case of emergency, so on and so forth. Moving passengers around affects all of those calculations, and especially when fuel margins are right at the limit -- I can see why they didn't want anyone moving around.

My point being ... there's a lot that goes on behind the scenes to prepare for a flight that you and I don't see.

Simply, if the pilots say "stay where you are" --- it's best to do so. W&B miscalculations can be ... catastrophic.