On BA you can pay for seating selections in advance, or no charge from all the available seats at online check-in beginning 23 hours in advance.
However, I am a bit confused by your post. I can't see where you could ever have flown Charlotte to Venice non-stop, this is a US Air route that went Charlotte to Philadelphia to Venice, and is still available in all its forms through American Airlines. In fact, depending on when you would fly this, the planes might not yet have been repainted with the AA logo. The effect of the AA-US Air merger on routes is minimal, other than the elimination of any duplicate flight between the two airlines.
I did a trial on BA for April and it gave the various AA flights to Venice that used to be US Air (and in fact show on the US Air reservation system as US Air flights until mid-October), and no direct flights. In fact, ita matrix states there are no direct flights to Venice from Charlotte. It does offer an interesting alternative, though - Lufthansa, with the one stop being in Munich. As you have no doubt seen many times here, having the one stop in Europe rather than in the US is always preferable, and I think all here would agree that a transfer in Munich is way preferable to a transfer in Philadelphia..
One practical difference between AA and US Air that I did notice - in early July we booked PHL-London, and return from Dublin, on US Air, for April 2016. US AIr had a much more liberal policy for seat selection, and in fact our row 18 seats on both flights which we were able to select for free would have required an additional payment of $70 per seat had we booked a month later when AA had taken over the reservations for next year. Their first free seats were much further back and in the middle.