If this is not the very first flight of the day for this plane, it's possible the flight will have passengers already on it continuing on from the inbound flight. Those passengers are free to switch seats before the next passengers (including pre-boards) get on and the first row seats may already be occupied when you get on. You might want to have a Plan B seat in mind - (would an aisle seat work for you if the booted foot was toward the aisle?