Which seats are held for check-in?

Greetings all,
My flight from LHR-JNB in PremEcon has just been changed from a 346 to a 744 (so I get the Upper Deck...woo hoo).
I'm planning on arriving at the airport first thing in the morning to check in my bags. Which seats are held back from online check-in that I may be able to choose from when I arrive at the airport?
Also, I'm currently in 74C - as I have never flown in PremEcon before - if I had to move to 74A, would I be able to get up without disturbing the person next to me?
Many thanks,
My flight from LHR-JNB in PremEcon has just been changed from a 346 to a 744 (so I get the Upper Deck...woo hoo).
I'm planning on arriving at the airport first thing in the morning to check in my bags. Which seats are held back from online check-in that I may be able to choose from when I arrive at the airport?
Also, I'm currently in 74C - as I have never flown in PremEcon before - if I had to move to 74A, would I be able to get up without disturbing the person next to me?
Many thanks,