The route-date can effect real taxes that are paid to governments~regulatory authorities. For example less than 24hrs between can have less taxes. The UK APD is a well know difference. So need a tax recalculation. But that does not change *ONE* ticket routing rules.
For AA155 to BA49 date/flight number change, on the same LHR-SEA segment, needs to be L class available. Seen reports some airlines (agents) want the $125 change fee for this. But some years ago I did a similar change at no cost. I would try AA again, as you have other AA segments. For AS *ONE* have read QF does the ticketing. But many QF phone agents are hopeless now, and would have little to no experience with *ONE* tickets.
http://www.gcmap.com/mapui?P=MEL-KUL...=bm&PW=3&DU=mi
You are only using 9 segments. Years ago I did a *ONE* that had a 9 month "stopover" in MEL. Basically got a trip to DRW for the cost of cheapie MEL-SYD & ADL-MEL flights. The *ONE* started in SYD and ended in ADL 363 days later.