Hi this isnt something ive ever seen before in the course of changing bookings and i suspect its because when you book it would upgrade both legs if you were booking a return journey. Please correct me if im wrong! If you were wanting to do it the way you're talking about you would need to book two singles the first through onbusiness the second a normal commercial fare to then upgrade with avios. This isnt going to be a cost effective way to book though because of the fare used for single tickets!

***** all opinions are my own and dont represent ba or iag******