If you are anywhere near reaching lifetime status with Qantas, i.e 7000 Status Credits for Lifetime Silver (Ruby) or 14000 Status Credits for Lifetime Gold (Sapphire) it might be worth considering continuing to credit to QF, even if you choose to fly JAL.
I'm about 2/3 the way to QF lifetime silver, which I hadn't thought about in this context. Thanks for flagging, worth adding to the equation.