ABPs and reseating.....
Me again, back from work!
Rule of thumb is you shouldn't move pax about just because they want to in an empty cabin. They've been seated where they are due to weight and balance. However, if your exit rows are empty, you will need pax in them regardless of weight and balance. On this instance, you would move people to your exit rows so you have an ABP. If people start whinging and moving about, you just say you're free to move seats after take off!
I did it today with a passenger (albeit on a A319 and not a 747-400) One of our overwings was empty, so I just moved a guy across! Job done!