Just have someone remove the lax-lon from the pnr and create a new one for that segment. Assign the QF number to the new pnr. Or just tell them at the gate after you get your boarding passes. Either will work fine. If youre worried about the second segment, just make sure the number is printed on the boarding pass and ask the gate agent what number is assigned to that segment. If it's wrong, the gate agent should be able to reassign it back.