Qantas Flight Date Change - original and new flight Still in booking

I've just changed the flight home of my Qantas return trip (Melb - Tokyo) through the call center and while my new flight has been added to my booking, the old flight is still in there.

I am now travelling back to Melbourne via Brisbane as there were no direct flights available on my new date (got some money refunded though surprisingly!).

My question is, will Qantas eventually remove the old flight from my booking at some point? I'm a little worried that if they do, any seat selections I make (and planed domestic leg upgrade) on the flights will be wiped. This already happened to me when I called up to only inquire about a possible date change. I'm traveling solo with a 2 year old to meet up with my wife on her business trip so seats are important! I want to try and avoid calling them again, the first agent I spoke to was hopeless, locked me out of my booking and wiped the seats I had paid for!

I appreciate you help.