The airline should advise if they are not the operating carrier - however where there is a short notice lease, it is very unusual to be told as they have more important priorities like getting you to your destination.
I have seen both the B733 and B734 and to be honest, the 733 does look a little tatty but in all honesty apart from those with the knowledge do we really think the average family will take any notice? A plane is a plane to them.